diff --git a/pkg/fsm/last_sign_state.go b/pkg/fsm/last_sign_state.go
index ba1edda..1886659 100644
--- a/pkg/fsm/last_sign_state.go
+++ b/pkg/fsm/last_sign_state.go
@@ -32,7 +32,13 @@ func (s *LastSignState) CopyToFilePV(pv *privval.FilePVLastSignState) {
 	if pv == nil {
 		return
 	}
-
+	if s.Less(&LastSignState{
+		Height: pv.Height,
+		Round:  pv.Round,
+		Step:   pv.Step,
+	}) {
+		return
+	}
 	pv.Height = s.Height
 	pv.Round = s.Round
 	pv.Step = s.Step
diff --git a/pkg/keyserver/signing_integration_test.go b/pkg/keyserver/signing_integration_test.go
index 8edc2e8..6276c51 100644
--- a/pkg/keyserver/signing_integration_test.go
+++ b/pkg/keyserver/signing_integration_test.go
@@ -426,6 +426,103 @@ func TestKeyserverMultipleValidatorsRejectConflictingVotes(t *testing.T) {
 	}
 }
 
+func TestKeyserverRejectsConflictingVotesAtSameHeight(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	peers := makeTestPeers(t, 3)
+	cluster := startRaftCluster(t, ctx, peers)
+	t.Cleanup(func() {
+		for _, node := range cluster {
+			closeClusterNode(t, node)
+		}
+	})
+
+	baseKeyDir := t.TempDir()
+	baseKeyPath := filepath.Join(baseKeyDir, "priv_key.json")
+	baseStatePath := filepath.Join(baseKeyDir, "priv_state.json")
+	basePV := privval.GenFilePV(baseKeyPath, baseStatePath)
+	basePV.Save()
+
+	validatorAddrA, endpointA, signerA := startValidator(t, testChainID)
+	validatorAddrB, endpointB, signerB := startValidator(t, testChainID)
+	t.Cleanup(func() {
+		if err := signerA.Close(); err != nil {
+			t.Logf("signer A close: %v", err)
+		}
+		if err := signerB.Close(); err != nil {
+			t.Logf("signer B close: %v", err)
+		}
+		endpointA.Stop()
+		endpointB.Stop()
+	})
+
+	startKeyservers(t, ctx, cluster, baseKeyPath, baseStatePath, []string{validatorAddrA, validatorAddrB}, testChainID)
+
+	_ = waitForLeaderNode(t, cluster, 10*time.Second)
+	waitForValidatorConnection(t, signerA, 10*time.Second)
+	waitForValidatorConnection(t, signerB, 10*time.Second)
+
+	validatorAddress := ed25519.GenPrivKey().PubKey().Address().Bytes()
+	height := int64(40)
+
+	conflictA := makeVote(height, 0x33, validatorAddress)
+	conflictB := makeVote(height, 0x44, validatorAddress)
+
+	var hookCount atomic.Int32
+	syncLastSignStateHook = func(point string, state *fsm.LastSignState) {
+		if point == "before-write" {
+			if res := hookCount.Add(1); res == 1 {
+				time.Sleep(time.Second)
+			}
+		}
+	}
+	t.Cleanup(func() {
+		syncLastSignStateHook = nil
+	})
+
+	results := make(chan error, 2)
+	start := make(chan struct{})
+	var wg sync.WaitGroup
+	wg.Add(2)
+
+	go func() {
+		defer wg.Done()
+		<-start
+		attempt := cloneVote(conflictA)
+		results <- signerA.SignVote(testChainID, attempt)
+	}()
+
+	go func() {
+		defer wg.Done()
+		<-start
+		attempt := cloneVote(conflictB)
+		results <- signerB.SignVote(testChainID, attempt)
+	}()
+
+	close(start)
+
+	wg.Wait()
+	close(results)
+
+	errCount := 0
+	for res := range results {
+		if res != nil {
+			var remoteErr *privval.RemoteSignerError
+			if !errors.As(res, &remoteErr) {
+				t.Fatalf("expected remote signer error, got %v", res)
+			}
+			if !strings.Contains(remoteErr.Description, "conflicting") {
+				t.Fatalf("unexpected error: %v", remoteErr)
+			}
+			errCount++
+		}
+	}
+	if errCount == 0 {
+		t.Fatal("expected at least one conflicting signature to fail")
+	}
+}
+
 func startKeyservers(t *testing.T, ctx context.Context, cluster []*clusterNode, baseKeyPath, baseStatePath string, validatorAddrs []string, chainID string) {
 	t.Helper()
 
diff --git a/pkg/keyserver/validator.go b/pkg/keyserver/validator.go
index ac028e3..f4efaaa 100644
--- a/pkg/keyserver/validator.go
+++ b/pkg/keyserver/validator.go
@@ -20,6 +20,10 @@ type PrivValidator struct {
 	mu    sync.Mutex
 }
 
+// syncLastSignStateHook is for tests to coordinate interleavings.
+// It should be nil in production.
+var syncLastSignStateHook func(point string, state *fsm.LastSignState)
+
 // NewPrivValidator returns a validator that defers to inner once the
 // CometKMS lease is available.
 func NewPrivValidator(inner *privval.FilePV, node *raftnode.Node) *PrivValidator {
@@ -80,21 +84,27 @@ func (l *PrivValidator) SignProposal(chainID string, proposal *cmtproto.Proposal
 // on-disk priv-validator state so leadership changes cannot re-sign old blocks.
 func (l *PrivValidator) syncLastSignState() error {
 	l.mu.Lock()
+	defer l.mu.Unlock()
+
 	lastSignState := fsm.FromFilePV(&l.inner.LastSignState)
 	if lastSignState.Equal(l.node.GetLastSignState()) {
-		l.mu.Unlock()
 		return nil
 	}
-	l.mu.Unlock()
+
+	if syncLastSignStateHook != nil {
+		syncLastSignStateHook("before-raft", lastSignState)
+	}
 
 	state, err := l.node.SyncLastSignState(lastSignState)
 	if err != nil {
 		return err
 	}
 
-	l.mu.Lock()
+	if syncLastSignStateHook != nil {
+		syncLastSignStateHook("before-write", state)
+	}
+
 	state.CopyToFilePV(&l.inner.LastSignState)
-	l.mu.Unlock()
 
 	return nil
 }