diff --git a/pkg/core/blockchain.go b/pkg/core/blockchain.go
index d22326398..32361f61f 100644
--- a/pkg/core/blockchain.go
+++ b/pkg/core/blockchain.go
@@ -431,20 +431,23 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 		}
 
 		// Process TX inputs that are grouped by previous hash.
-		for prevHash, inputs := range tx.GroupInputsByPrevHash() {
+		for _, inputs := range transaction.GroupInputsByPrevHash(tx.Inputs) {
+			prevHash := inputs[0].PrevHash
 			prevTX, prevTXHeight, err := bc.dao.GetTransaction(prevHash)
 			if err != nil {
 				return fmt.Errorf("could not find previous TX: %s", prevHash)
 			}
+			unspent, err := cache.GetUnspentCoinStateOrNew(prevHash)
+			if err != nil {
+				return err
+			}
+			spentCoin, err := cache.GetSpentCoinsOrNew(prevHash, prevTXHeight)
+			if err != nil {
+				return err
+			}
+			oldSpentCoinLen := len(spentCoin.items)
 			for _, input := range inputs {
-				unspent, err := cache.GetUnspentCoinStateOrNew(input.PrevHash)
-				if err != nil {
-					return err
-				}
 				unspent.states[input.PrevIndex] = state.CoinSpent
-				if err = cache.PutUnspentCoinState(input.PrevHash, unspent); err != nil {
-					return err
-				}
 				prevTXOutput := prevTX.Outputs[input.PrevIndex]
 				account, err := cache.GetAccountStateOrNew(prevTXOutput.ScriptHash)
 				if err != nil {
@@ -452,11 +455,7 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 				}
 
 				if prevTXOutput.AssetID.Equals(GoverningTokenID()) {
-					spentCoin := NewSpentCoinState(input.PrevHash, prevTXHeight)
 					spentCoin.items[input.PrevIndex] = block.Index
-					if err = cache.PutSpentCoinState(input.PrevHash, spentCoin); err != nil {
-						return err
-					}
 					if err = processTXWithValidatorsSubtract(&prevTXOutput, account, cache); err != nil {
 						return err
 					}
@@ -482,6 +481,14 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 					return err
 				}
 			}
+			if err = cache.PutUnspentCoinState(prevHash, unspent); err != nil {
+				return err
+			}
+			if oldSpentCoinLen != len(spentCoin.items) {
+				if err = cache.PutSpentCoinState(prevHash, spentCoin); err != nil {
+					return err
+				}
+			}
 		}
 
 		// Process the underlying type of the TX.
@@ -517,18 +524,34 @@ func (bc *Blockchain) storeBlock(block *block.Block) error {
 			// Remove claimed NEO from spent coins making it unavalaible for
 			// additional claims.
 			for _, input := range t.Claims {
-				scs, err := cache.GetSpentCoinsOrNew(input.PrevHash)
-				if err != nil {
-					return err
+				scs, err := cache.GetSpentCoinState(input.PrevHash)
+				if err == nil {
+					_, ok := scs.items[input.PrevIndex]
+					if !ok {
+						err = errors.New("no spent coin state")
+					}
 				}
-				if scs.txHash == input.PrevHash {
-					// Existing scs.
-					delete(scs.items, input.PrevIndex)
+				if err != nil {
+					// We can't really do anything about it
+					// as it's a transaction in a signed block.
+					bc.log.Warn("DOUBLE CLAIM",
+						zap.String("PrevHash", input.PrevHash.StringLE()),
+						zap.Uint16("PrevIndex", input.PrevIndex),
+						zap.String("tx", tx.Hash().StringLE()),
+						zap.Uint32("block", block.Index),
+					)
+					// "Strict" mode.
+					if bc.config.VerifyTransactions {
+						return err
+					}
+					break
+				}
+				delete(scs.items, input.PrevIndex)
+				if len(scs.items) > 0 {
 					if err = cache.PutSpentCoinState(input.PrevHash, scs); err != nil {
 						return err
 					}
 				} else {
-					// Uninitialized, new, forget about it.
 					if err = cache.DeleteSpentCoinState(input.PrevHash); err != nil {
 						return err
 					}
@@ -986,27 +1009,34 @@ func (bc *Blockchain) GetConfig() config.ProtocolConfiguration {
 	return bc.config
 }
 
-// References returns a map with input coin reference (prevhash and index) as key
-// and transaction output as value from a transaction t.
+// References maps transaction's inputs into a slice of InOuts, effectively
+// joining each Input with the corresponding Output.
 // @TODO: unfortunately we couldn't attach this method to the Transaction struct in the
 // transaction package because of a import cycle problem. Perhaps we should think to re-design
 // the code base to avoid this situation.
-func (bc *Blockchain) References(t *transaction.Transaction) map[transaction.Input]*transaction.Output {
-	references := make(map[transaction.Input]*transaction.Output)
+func (bc *Blockchain) References(t *transaction.Transaction) ([]transaction.InOut, error) {
+	return bc.references(t.Inputs)
+}
 
-	for prevHash, inputs := range t.GroupInputsByPrevHash() {
+// references is an internal implementation of References that operates directly
+// on a slice of Input.
+func (bc *Blockchain) references(ins []transaction.Input) ([]transaction.InOut, error) {
+	references := make([]transaction.InOut, 0, len(ins))
+
+	for _, inputs := range transaction.GroupInputsByPrevHash(ins) {
+		prevHash := inputs[0].PrevHash
 		tx, _, err := bc.dao.GetTransaction(prevHash)
 		if err != nil {
-			return nil
+			return nil, errors.New("bad input reference")
 		}
 		for _, in := range inputs {
 			if int(in.PrevIndex) > len(tx.Outputs)-1 {
-				return nil
+				return nil, errors.New("bad input reference")
 			}
-			references[*in] = &tx.Outputs[in.PrevIndex]
+			references = append(references, transaction.InOut{In: *in, Out: tx.Outputs[in.PrevIndex]})
 		}
 	}
-	return references
+	return references, nil
 }
 
 // FeePerByte returns network fee divided by the size of the transaction.
@@ -1017,16 +1047,20 @@ func (bc *Blockchain) FeePerByte(t *transaction.Transaction) util.Fixed8 {
 // NetworkFee returns network fee.
 func (bc *Blockchain) NetworkFee(t *transaction.Transaction) util.Fixed8 {
 	inputAmount := util.Fixed8FromInt64(0)
-	for _, txOutput := range bc.References(t) {
-		if txOutput.AssetID == UtilityTokenID() {
-			inputAmount.Add(txOutput.Amount)
+	refs, err := bc.References(t)
+	if err != nil {
+		return inputAmount
+	}
+	for i := range refs {
+		if refs[i].Out.AssetID == UtilityTokenID() {
+			inputAmount = inputAmount.Add(refs[i].Out.Amount)
 		}
 	}
 
 	outputAmount := util.Fixed8FromInt64(0)
 	for _, txOutput := range t.Outputs {
 		if txOutput.AssetID == UtilityTokenID() {
-			outputAmount.Add(txOutput.Amount)
+			outputAmount = outputAmount.Add(txOutput.Amount)
 		}
 	}
 
@@ -1087,7 +1121,7 @@ func (bc *Blockchain) verifyTx(t *transaction.Transaction, block *block.Block) e
 	if io.GetVarSize(t) > transaction.MaxTransactionSize {
 		return errors.Errorf("invalid transaction size = %d. It shoud be less then MaxTransactionSize = %d", io.GetVarSize(t), transaction.MaxTransactionSize)
 	}
-	if ok := bc.verifyInputs(t); !ok {
+	if transaction.HaveDuplicateInputs(t.Inputs) {
 		return errors.New("invalid transaction's inputs")
 	}
 	if block == nil {
@@ -1111,6 +1145,16 @@ func (bc *Blockchain) verifyTx(t *transaction.Transaction, block *block.Block) e
 		}
 	}
 
+	if t.Type == transaction.ClaimType {
+		claim := t.Data.(*transaction.ClaimTX)
+		if transaction.HaveDuplicateInputs(claim.Claims) {
+			return errors.New("duplicate claims")
+		}
+		if bc.dao.IsDoubleClaim(claim) {
+			return errors.New("double claim")
+		}
+	}
+
 	return bc.verifyTxWitnesses(t, block)
 }
 
@@ -1130,6 +1174,12 @@ func (bc *Blockchain) isTxStillRelevant(t *transaction.Transaction) bool {
 	if bc.dao.IsDoubleSpend(t) {
 		return false
 	}
+	if t.Type == transaction.ClaimType {
+		claim := t.Data.(*transaction.ClaimTX)
+		if bc.dao.IsDoubleClaim(claim) {
+			return false
+		}
+	}
 	for i := range t.Scripts {
 		if !vm.IsStandardContract(t.Scripts[i].VerificationScript) {
 			recheckWitness = true
@@ -1189,18 +1239,6 @@ func (bc *Blockchain) PoolTx(t *transaction.Transaction) error {
 	return nil
 }
 
-func (bc *Blockchain) verifyInputs(t *transaction.Transaction) bool {
-	for i := 1; i < len(t.Inputs); i++ {
-		for j := 0; j < i; j++ {
-			if t.Inputs[i].PrevHash == t.Inputs[j].PrevHash && t.Inputs[i].PrevIndex == t.Inputs[j].PrevIndex {
-				return false
-			}
-		}
-	}
-
-	return true
-}
-
 func (bc *Blockchain) verifyOutputs(t *transaction.Transaction) error {
 	for assetID, outputs := range t.GroupOutputByAssetID() {
 		assetState := bc.GetAssetState(assetID)
@@ -1286,14 +1324,14 @@ func (bc *Blockchain) GetTransactionResults(t *transaction.Transaction) []*trans
 	var results []*transaction.Result
 	tempGroupResult := make(map[util.Uint256]util.Fixed8)
 
-	references := bc.References(t)
-	if references == nil {
+	references, err := bc.References(t)
+	if err != nil {
 		return nil
 	}
-	for _, output := range references {
+	for _, inout := range references {
 		tempResults = append(tempResults, &transaction.Result{
-			AssetID: output.AssetID,
-			Amount:  output.Amount,
+			AssetID: inout.Out.AssetID,
+			Amount:  inout.Out.Amount,
 		})
 	}
 	for _, output := range t.Outputs {
@@ -1323,39 +1361,6 @@ func (bc *Blockchain) GetTransactionResults(t *transaction.Transaction) []*trans
 	return results
 }
 
-// GetScriptHashesForVerifyingClaim returns all ScriptHashes of Claim transaction
-// which has a different implementation from generic GetScriptHashesForVerifying.
-func (bc *Blockchain) GetScriptHashesForVerifyingClaim(t *transaction.Transaction) ([]util.Uint160, error) {
-	// Avoiding duplicates.
-	hashmap := make(map[util.Uint160]bool)
-
-	claim := t.Data.(*transaction.ClaimTX)
-	clGroups := make(map[util.Uint256][]*transaction.Input)
-	for _, in := range claim.Claims {
-		clGroups[in.PrevHash] = append(clGroups[in.PrevHash], in)
-	}
-	for group, inputs := range clGroups {
-		refTx, _, err := bc.dao.GetTransaction(group)
-		if err != nil {
-			return nil, err
-		}
-		for _, input := range inputs {
-			if len(refTx.Outputs) <= int(input.PrevIndex) {
-				return nil, fmt.Errorf("wrong PrevIndex reference")
-			}
-			hashmap[refTx.Outputs[input.PrevIndex].ScriptHash] = true
-		}
-	}
-	if len(hashmap) > 0 {
-		hashes := make([]util.Uint160, 0, len(hashmap))
-		for k := range hashmap {
-			hashes = append(hashes, k)
-		}
-		return hashes, nil
-	}
-	return nil, fmt.Errorf("no hashes found")
-}
-
 //GetStandByValidators returns validators from the configuration.
 func (bc *Blockchain) GetStandByValidators() (keys.PublicKeys, error) {
 	return getValidators(bc.config)
@@ -1507,19 +1512,13 @@ func processEnrollmentTX(dao *cachedDao, tx *transaction.EnrollmentTX) error {
 // to verify whether the transaction is bonafide or not.
 // Golang implementation of GetScriptHashesForVerifying method in C# (https://github.com/neo-project/neo/blob/master/neo/Network/P2P/Payloads/Transaction.cs#L190)
 func (bc *Blockchain) GetScriptHashesForVerifying(t *transaction.Transaction) ([]util.Uint160, error) {
-	if t.Type == transaction.ClaimType {
-		return bc.GetScriptHashesForVerifyingClaim(t)
-	}
-	references := bc.References(t)
-	if references == nil {
-		return nil, errors.New("invalid inputs")
+	references, err := bc.References(t)
+	if err != nil {
+		return nil, err
 	}
 	hashes := make(map[util.Uint160]bool)
-	for _, i := range t.Inputs {
-		h := references[i].ScriptHash
-		if _, ok := hashes[h]; !ok {
-			hashes[h] = true
-		}
+	for i := range references {
+		hashes[references[i].Out.ScriptHash] = true
 	}
 	for _, a := range t.Attributes {
 		if a.Usage == transaction.Script {
@@ -1547,6 +1546,20 @@ func (bc *Blockchain) GetScriptHashesForVerifying(t *transaction.Transaction) ([
 			}
 		}
 	}
+	switch t.Type {
+	case transaction.ClaimType:
+		claim := t.Data.(*transaction.ClaimTX)
+		refs, err := bc.references(claim.Claims)
+		if err != nil {
+			return nil, err
+		}
+		for i := range refs {
+			hashes[refs[i].Out.ScriptHash] = true
+		}
+	case transaction.EnrollmentType:
+		etx := t.Data.(*transaction.EnrollmentTX)
+		hashes[etx.PublicKey.GetScriptHash()] = true
+	}
 	// convert hashes to []util.Uint160
 	hashesResult := make([]util.Uint160, 0, len(hashes))
 	for h := range hashes {
diff --git a/pkg/core/blockchainer.go b/pkg/core/blockchainer.go
index dae78c48b..822c80fec 100644
--- a/pkg/core/blockchainer.go
+++ b/pkg/core/blockchainer.go
@@ -39,7 +39,7 @@ type Blockchainer interface {
 	GetTestVM() (*vm.VM, storage.Store)
 	GetTransaction(util.Uint256) (*transaction.Transaction, uint32, error)
 	GetUnspentCoinState(util.Uint256) *UnspentCoinState
-	References(t *transaction.Transaction) map[transaction.Input]*transaction.Output
+	References(t *transaction.Transaction) ([]transaction.InOut, error)
 	mempool.Feer // fee interface
 	PoolTx(*transaction.Transaction) error
 	VerifyTx(*transaction.Transaction, *block.Block) error
diff --git a/pkg/core/dao.go b/pkg/core/dao.go
index 1ff5d5e5e..d84e50816 100644
--- a/pkg/core/dao.go
+++ b/pkg/core/dao.go
@@ -175,15 +175,13 @@ func (dao *dao) PutUnspentCoinState(hash util.Uint256, ucs *UnspentCoinState) er
 // -- start spent coins.
 
 // GetSpentCoinsOrNew returns spent coins from store.
-func (dao *dao) GetSpentCoinsOrNew(hash util.Uint256) (*SpentCoinState, error) {
+func (dao *dao) GetSpentCoinsOrNew(hash util.Uint256, height uint32) (*SpentCoinState, error) {
 	spent, err := dao.GetSpentCoinState(hash)
 	if err != nil {
 		if err != storage.ErrKeyNotFound {
 			return nil, err
 		}
-		spent = &SpentCoinState{
-			items: make(map[uint16]uint32),
-		}
+		spent = NewSpentCoinState(hash, height)
 	}
 	return spent, nil
 }
@@ -549,7 +547,8 @@ func (dao *dao) IsDoubleSpend(tx *transaction.Transaction) bool {
 	if len(tx.Inputs) == 0 {
 		return false
 	}
-	for prevHash, inputs := range tx.GroupInputsByPrevHash() {
+	for _, inputs := range transaction.GroupInputsByPrevHash(tx.Inputs) {
+		prevHash := inputs[0].PrevHash
 		unspent, err := dao.GetUnspentCoinState(prevHash)
 		if err != nil {
 			return false
@@ -563,6 +562,27 @@ func (dao *dao) IsDoubleSpend(tx *transaction.Transaction) bool {
 	return false
 }
 
+// IsDoubleClaim verifies that given claim inputs are not already claimed by another tx.
+func (dao *dao) IsDoubleClaim(claim *transaction.ClaimTX) bool {
+	if len(claim.Claims) == 0 {
+		return false
+	}
+	for _, inputs := range transaction.GroupInputsByPrevHash(claim.Claims) {
+		prevHash := inputs[0].PrevHash
+		scs, err := dao.GetSpentCoinState(prevHash)
+		if err != nil {
+			return true
+		}
+		for _, input := range inputs {
+			_, ok := scs.items[input.PrevIndex]
+			if !ok {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 // Persist flushes all the changes made into the (supposedly) persistent
 // underlying store.
 func (dao *dao) Persist() (int, error) {
diff --git a/pkg/core/dao_test.go b/pkg/core/dao_test.go
index 52df535bc..3b4a30343 100644
--- a/pkg/core/dao_test.go
+++ b/pkg/core/dao_test.go
@@ -124,7 +124,7 @@ func TestPutGetUnspentCoinState(t *testing.T) {
 func TestGetSpentCoinStateOrNew_New(t *testing.T) {
 	dao := newDao(storage.NewMemoryStore())
 	hash := random.Uint256()
-	spentCoinState, err := dao.GetSpentCoinsOrNew(hash)
+	spentCoinState, err := dao.GetSpentCoinsOrNew(hash, 1)
 	require.NoError(t, err)
 	require.NotNil(t, spentCoinState)
 }
diff --git a/pkg/core/interop_neo.go b/pkg/core/interop_neo.go
index b3018e51c..1eb8e9b3c 100644
--- a/pkg/core/interop_neo.go
+++ b/pkg/core/interop_neo.go
@@ -34,12 +34,6 @@ const (
 	DefaultAssetLifetime = 1 + BlocksPerYear
 )
 
-// txInOut is used to pushed one key-value pair from References() onto the stack.
-type txInOut struct {
-	in  transaction.Input
-	out transaction.Output
-}
-
 // headerGetVersion returns version from the header.
 func (ic *interopContext) headerGetVersion(v *vm.VM) error {
 	header, err := popHeaderFromVM(v)
@@ -141,14 +135,16 @@ func (ic *interopContext) txGetReferences(v *vm.VM) error {
 	if !ok {
 		return fmt.Errorf("type mismatch: %T is not a Transaction", txInterface)
 	}
-	refs := ic.bc.References(tx)
+	refs, err := ic.bc.References(tx)
+	if err != nil {
+		return err
+	}
 	if len(refs) > vm.MaxArraySize {
 		return errors.New("too many references")
 	}
 
 	stackrefs := make([]vm.StackItem, 0, len(refs))
-	for _, k := range tx.Inputs {
-		tio := txInOut{k, *refs[k]}
+	for _, tio := range refs {
 		stackrefs = append(stackrefs, vm.NewInteropItem(tio))
 	}
 	v.Estack().PushVal(stackrefs)
@@ -243,11 +239,11 @@ func popInputFromVM(v *vm.VM) (*transaction.Input, error) {
 	inInterface := v.Estack().Pop().Value()
 	input, ok := inInterface.(*transaction.Input)
 	if !ok {
-		txio, ok := inInterface.(txInOut)
+		txio, ok := inInterface.(transaction.InOut)
 		if !ok {
-			return nil, fmt.Errorf("type mismatch: %T is not an Input or txInOut", inInterface)
+			return nil, fmt.Errorf("type mismatch: %T is not an Input or InOut", inInterface)
 		}
-		input = &txio.in
+		input = &txio.In
 	}
 	return input, nil
 }
@@ -277,11 +273,11 @@ func popOutputFromVM(v *vm.VM) (*transaction.Output, error) {
 	outInterface := v.Estack().Pop().Value()
 	output, ok := outInterface.(*transaction.Output)
 	if !ok {
-		txio, ok := outInterface.(txInOut)
+		txio, ok := outInterface.(transaction.InOut)
 		if !ok {
-			return nil, fmt.Errorf("type mismatch: %T is not an Output or txInOut", outInterface)
+			return nil, fmt.Errorf("type mismatch: %T is not an Output or InOut", outInterface)
 		}
-		output = &txio.out
+		output = &txio.Out
 	}
 	return output, nil
 }
diff --git a/pkg/core/transaction/claim.go b/pkg/core/transaction/claim.go
index 423a4bc79..05f9b1a16 100644
--- a/pkg/core/transaction/claim.go
+++ b/pkg/core/transaction/claim.go
@@ -6,7 +6,7 @@ import (
 
 // ClaimTX represents a claim transaction.
 type ClaimTX struct {
-	Claims []*Input
+	Claims []Input
 }
 
 // DecodeBinary implements Serializable interface.
diff --git a/pkg/core/transaction/inout.go b/pkg/core/transaction/inout.go
new file mode 100644
index 000000000..75bc026ac
--- /dev/null
+++ b/pkg/core/transaction/inout.go
@@ -0,0 +1,8 @@
+package transaction
+
+// InOut represents an Input bound to its corresponding Output which is a useful
+// combination for many purposes.
+type InOut struct {
+	In  Input
+	Out Output
+}
diff --git a/pkg/core/transaction/input.go b/pkg/core/transaction/input.go
index da390cfe5..3b7758ba3 100644
--- a/pkg/core/transaction/input.go
+++ b/pkg/core/transaction/input.go
@@ -1,6 +1,8 @@
 package transaction
 
 import (
+	"sort"
+
 	"github.com/CityOfZion/neo-go/pkg/io"
 	"github.com/CityOfZion/neo-go/pkg/util"
 )
@@ -25,3 +27,62 @@ func (in *Input) EncodeBinary(bw *io.BinWriter) {
 	bw.WriteBytes(in.PrevHash[:])
 	bw.WriteU16LE(in.PrevIndex)
 }
+
+// MapInputsToSorted maps given slice of inputs into a new slice of pointers
+// to inputs sorted by their PrevHash and PrevIndex.
+func MapInputsToSorted(ins []Input) []*Input {
+	ptrs := make([]*Input, len(ins))
+	for i := range ins {
+		ptrs[i] = &ins[i]
+	}
+	sort.Slice(ptrs, func(i, j int) bool {
+		hashcmp := ptrs[i].PrevHash.CompareTo(ptrs[j].PrevHash)
+		if hashcmp == 0 {
+			return ptrs[i].PrevIndex < ptrs[j].PrevIndex
+		}
+		return hashcmp < 0
+	})
+	return ptrs
+}
+
+// GroupInputsByPrevHash groups all TX inputs by their previous hash into
+// several slices (which actually are subslices of one new slice with pointers).
+// Each of these slices contains at least one element.
+func GroupInputsByPrevHash(ins []Input) [][]*Input {
+	if len(ins) == 0 {
+		return nil
+	}
+
+	ptrs := MapInputsToSorted(ins)
+	var first int
+	res := make([][]*Input, 0)
+	currentHash := ptrs[0].PrevHash
+
+	for i := range ptrs {
+		if !currentHash.Equals(ptrs[i].PrevHash) {
+			res = append(res, ptrs[first:i])
+			first = i
+			currentHash = ptrs[i].PrevHash
+		}
+	}
+	res = append(res, ptrs[first:])
+	return res
+}
+
+// HaveDuplicateInputs checks inputs for duplicates and returns true if there are
+// any.
+func HaveDuplicateInputs(ins []Input) bool {
+	if len(ins) < 2 {
+		return false
+	}
+	if len(ins) == 2 {
+		return ins[0] == ins[1]
+	}
+	ptrs := MapInputsToSorted(ins)
+	for i := 1; i < len(ptrs); i++ {
+		if *ptrs[i] == *ptrs[i-1] {
+			return true
+		}
+	}
+	return false
+}
diff --git a/pkg/core/transaction/input_test.go b/pkg/core/transaction/input_test.go
new file mode 100644
index 000000000..0375a4559
--- /dev/null
+++ b/pkg/core/transaction/input_test.go
@@ -0,0 +1,144 @@
+package transaction
+
+import (
+	"testing"
+
+	"github.com/CityOfZion/neo-go/pkg/util"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGroupInputsByPrevHash0(t *testing.T) {
+	inputs := make([]Input, 0)
+	res := GroupInputsByPrevHash(inputs)
+	require.Equal(t, 0, len(res))
+}
+
+func TestGroupInputsByPrevHash1(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash, err := util.Uint256DecodeStringLE("46168f963d6d8168a870405f66cc9e13a235791013b8ee2f90cc20a8293bd1af")
+	require.NoError(t, err)
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 42})
+	res := GroupInputsByPrevHash(inputs)
+	require.Equal(t, 1, len(res))
+	require.Equal(t, 1, len(res[0]))
+	assert.Equal(t, hash, res[0][0].PrevHash)
+	assert.Equal(t, uint16(42), res[0][0].PrevIndex)
+}
+
+func TestGroupInputsByPrevHashMany(t *testing.T) {
+	hash1, err := util.Uint256DecodeStringBE("a83ba6ede918a501558d3170a124324aedc89909e64c4ff2c6f863094f980b25")
+	require.NoError(t, err)
+	hash2, err := util.Uint256DecodeStringBE("629397158f852e838077bb2715b13a2e29b0a51c2157e5466321b70ed7904ce9")
+	require.NoError(t, err)
+	hash3, err := util.Uint256DecodeStringBE("caa41245c3e48ddc13dabe989ba8fbc59418e9228fef9efb62855b0b17d7448b")
+	require.NoError(t, err)
+	inputs := make([]Input, 0)
+	for i := 0; i < 10; i++ {
+		inputs = append(inputs, Input{PrevHash: hash1, PrevIndex: uint16(i)})
+		inputs = append(inputs, Input{PrevHash: hash2, PrevIndex: uint16(i)})
+		inputs = append(inputs, Input{PrevHash: hash3, PrevIndex: uint16(i)})
+	}
+	for i := 15; i < 20; i++ {
+		inputs = append(inputs, Input{PrevHash: hash3, PrevIndex: uint16(i)})
+	}
+	for i := 10; i < 15; i++ {
+		inputs = append(inputs, Input{PrevHash: hash2, PrevIndex: uint16(i)})
+		inputs = append(inputs, Input{PrevHash: hash3, PrevIndex: uint16(i)})
+	}
+	seen := make(map[uint16]bool)
+	res := GroupInputsByPrevHash(inputs)
+	require.Equal(t, 3, len(res))
+	assert.Equal(t, hash2, res[0][0].PrevHash)
+	assert.Equal(t, 15, len(res[0]))
+	for i := range res[0] {
+		assert.Equal(t, res[0][i].PrevHash, res[0][0].PrevHash)
+		assert.Equal(t, false, seen[res[0][i].PrevIndex])
+		seen[res[0][i].PrevIndex] = true
+	}
+	seen = make(map[uint16]bool)
+	assert.Equal(t, hash1, res[1][0].PrevHash)
+	assert.Equal(t, 10, len(res[1]))
+	for i := range res[1] {
+		assert.Equal(t, res[1][i].PrevHash, res[1][0].PrevHash)
+		assert.Equal(t, false, seen[res[1][i].PrevIndex])
+		seen[res[1][i].PrevIndex] = true
+	}
+	seen = make(map[uint16]bool)
+	assert.Equal(t, hash3, res[2][0].PrevHash)
+	assert.Equal(t, 20, len(res[2]))
+	for i := range res[2] {
+		assert.Equal(t, res[2][i].PrevHash, res[2][0].PrevHash)
+		assert.Equal(t, false, seen[res[2][i].PrevIndex])
+		seen[res[2][i].PrevIndex] = true
+	}
+}
+
+func TestHaveDuplicateInputs0(t *testing.T) {
+	inputs := make([]Input, 0)
+	require.False(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputs1(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash, err := util.Uint256DecodeStringLE("46168f963d6d8168a870405f66cc9e13a235791013b8ee2f90cc20a8293bd1af")
+	require.NoError(t, err)
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 42})
+	require.False(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputs2True(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash, err := util.Uint256DecodeStringLE("46168f963d6d8168a870405f66cc9e13a235791013b8ee2f90cc20a8293bd1af")
+	require.NoError(t, err)
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 42})
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 42})
+	require.True(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputs2FalseInd(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash, err := util.Uint256DecodeStringLE("46168f963d6d8168a870405f66cc9e13a235791013b8ee2f90cc20a8293bd1af")
+	require.NoError(t, err)
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 42})
+	inputs = append(inputs, Input{PrevHash: hash, PrevIndex: 41})
+	require.False(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputs2FalseHash(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash1, err := util.Uint256DecodeStringBE("a83ba6ede918a501558d3170a124324aedc89909e64c4ff2c6f863094f980b25")
+	require.NoError(t, err)
+	hash2, err := util.Uint256DecodeStringBE("629397158f852e838077bb2715b13a2e29b0a51c2157e5466321b70ed7904ce9")
+	require.NoError(t, err)
+	inputs = append(inputs, Input{PrevHash: hash1, PrevIndex: 42})
+	inputs = append(inputs, Input{PrevHash: hash2, PrevIndex: 42})
+	require.False(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputsMFalse(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash1, err := util.Uint256DecodeStringBE("a83ba6ede918a501558d3170a124324aedc89909e64c4ff2c6f863094f980b25")
+	require.NoError(t, err)
+	hash2, err := util.Uint256DecodeStringBE("629397158f852e838077bb2715b13a2e29b0a51c2157e5466321b70ed7904ce9")
+	require.NoError(t, err)
+	for i := 0; i < 10; i++ {
+		inputs = append(inputs, Input{PrevHash: hash1, PrevIndex: uint16(i)})
+		inputs = append(inputs, Input{PrevHash: hash2, PrevIndex: uint16(i)})
+	}
+	require.False(t, HaveDuplicateInputs(inputs))
+}
+
+func TestHaveDuplicateInputsMTrue(t *testing.T) {
+	inputs := make([]Input, 0)
+	hash1, err := util.Uint256DecodeStringBE("a83ba6ede918a501558d3170a124324aedc89909e64c4ff2c6f863094f980b25")
+	require.NoError(t, err)
+	hash2, err := util.Uint256DecodeStringBE("629397158f852e838077bb2715b13a2e29b0a51c2157e5466321b70ed7904ce9")
+	require.NoError(t, err)
+	for i := 0; i < 10; i++ {
+		inputs = append(inputs, Input{PrevHash: hash1, PrevIndex: uint16(i)})
+		inputs = append(inputs, Input{PrevHash: hash2, PrevIndex: uint16(i)})
+	}
+	inputs = append(inputs, Input{PrevHash: hash1, PrevIndex: 0})
+	require.True(t, HaveDuplicateInputs(inputs))
+}
diff --git a/pkg/core/transaction/transaction.go b/pkg/core/transaction/transaction.go
index 46c0bee77..cdabfa3c0 100644
--- a/pkg/core/transaction/transaction.go
+++ b/pkg/core/transaction/transaction.go
@@ -187,16 +187,6 @@ func (t *Transaction) createHash() error {
 	return nil
 }
 
-// GroupInputsByPrevHash groups all TX inputs by their previous hash.
-func (t *Transaction) GroupInputsByPrevHash() map[util.Uint256][]*Input {
-	m := make(map[util.Uint256][]*Input)
-	for i := range t.Inputs {
-		hash := t.Inputs[i].PrevHash
-		m[hash] = append(m[hash], &t.Inputs[i])
-	}
-	return m
-}
-
 // GroupOutputByAssetID groups all TX outputs by their assetID.
 func (t Transaction) GroupOutputByAssetID() map[util.Uint256][]*Output {
 	m := make(map[util.Uint256][]*Output)
diff --git a/pkg/network/helper_test.go b/pkg/network/helper_test.go
index 2759c37df..d21eb2ae6 100644
--- a/pkg/network/helper_test.go
+++ b/pkg/network/helper_test.go
@@ -33,7 +33,7 @@ func (chain testChain) GetConfig() config.ProtocolConfiguration {
 	panic("TODO")
 }
 
-func (chain testChain) References(t *transaction.Transaction) map[transaction.Input]*transaction.Output {
+func (chain testChain) References(t *transaction.Transaction) ([]transaction.InOut, error) {
 	panic("TODO")
 }