mempool: swap checks in Add, fail fast

Checking for duplicates is easier than checking the balance, so it should be
done first.

pkg/core/mempool/mem_pool.go

@@ -136,14 +136,14 @@ func (mp *Pool) Add(t *transaction.Transaction, fee Feer) error {
 		timeStamp: time.Now().UTC(),
 	}
 	mp.lock.Lock()
-	if !mp.checkTxConflicts(t, fee) {
-		mp.lock.Unlock()
-		return ErrConflict
-	}
 	if mp.containsKey(t.Hash()) {
 		mp.lock.Unlock()
 		return ErrDup
 	}
+	if !mp.checkTxConflicts(t, fee) {
+		mp.lock.Unlock()
+		return ErrConflict
+	}
 
 	mp.verifiedMap[t.Hash()] = pItem
 	// Insert into sorted array (from max to min, that could also be done