diff --git a/repository/master_index_test.go b/repository/master_index_test.go
new file mode 100644
index 000000000..923eef338
--- /dev/null
+++ b/repository/master_index_test.go
@@ -0,0 +1,146 @@
+package repository
+
+import (
+	"bytes"
+	"crypto/rand"
+	"errors"
+	"io"
+	mrand "math/rand"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/restic/restic/backend"
+	"github.com/restic/restic/pack"
+)
+
+const parallelSaves = 20
+const saveIndexTime = 100 * time.Millisecond
+const testTimeout = 1 * time.Second
+
+var DupID backend.ID
+
+func randomID() backend.ID {
+	if mrand.Float32() < 0.5 {
+		return DupID
+	}
+
+	id := backend.ID{}
+	_, err := io.ReadFull(rand.Reader, id[:])
+	if err != nil {
+		panic(err)
+	}
+	return id
+}
+
+// forgetfulBackend returns a backend that forgets everything.
+func forgetfulBackend() backend.Backend {
+	be := &backend.MockBackend{}
+
+	be.TestFn = func(t backend.Type, name string) (bool, error) {
+		return false, nil
+	}
+
+	be.LoadFn = func(h backend.Handle, p []byte, off int64) (int, error) {
+		return 0, errors.New("not found")
+	}
+
+	be.SaveFn = func(h backend.Handle, p []byte) error {
+		return nil
+	}
+
+	be.StatFn = func(h backend.Handle) (backend.BlobInfo, error) {
+		return backend.BlobInfo{}, errors.New("not found")
+	}
+
+	be.RemoveFn = func(t backend.Type, name string) error {
+		return nil
+	}
+
+	be.ListFn = func(t backend.Type, done <-chan struct{}) <-chan string {
+		ch := make(chan string)
+		close(ch)
+		return ch
+	}
+
+	be.DeleteFn = func() error {
+		return nil
+	}
+
+	return be
+}
+
+func testMasterIndex(t *testing.T) {
+	_, err := io.ReadFull(rand.Reader, DupID[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	repo := New(forgetfulBackend())
+	err = repo.Init("foo")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wg := &sync.WaitGroup{}
+	done := make(chan struct{})
+	for i := 0; i < parallelSaves; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for {
+				select {
+				case <-done:
+					return
+				default:
+				}
+
+				id := randomID()
+
+				if repo.Index().Has(id) {
+					continue
+				}
+
+				buf := make([]byte, 50)
+
+				err := repo.SaveFrom(pack.Data, &id, uint(len(buf)), bytes.NewReader(buf))
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+		}()
+	}
+
+	saveIndexes := func() {
+		defer wg.Done()
+
+		ticker := time.NewTicker(saveIndexTime)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-done:
+				return
+			case <-ticker.C:
+				err := repo.SaveFullIndex()
+				if err != nil {
+					t.Fatal(err)
+				}
+			}
+		}
+	}
+
+	wg.Add(1)
+	go saveIndexes()
+
+	<-time.After(testTimeout)
+	close(done)
+
+	wg.Wait()
+}
+
+func TestMasterIndex(t *testing.T) {
+	for i := 0; i < 5; i++ {
+		testMasterIndex(t)
+	}
+}