diff --git a/keyserver/internal/device_list_update_test.go b/keyserver/internal/device_list_update_test.go
index 164be6be..e8d1bfe8 100644
--- a/keyserver/internal/device_list_update_test.go
+++ b/keyserver/internal/device_list_update_test.go
@@ -48,13 +48,19 @@ type mockDeviceListUpdaterDatabase struct {
 	staleUsers   map[string]bool
 	prevIDsExist func(string, []int) bool
 	storedKeys   []api.DeviceMessage
+	mu           sync.Mutex // protect staleUsers
 }
 
 // StaleDeviceLists returns a list of user IDs ending with the domains provided who have stale device lists.
 // If no domains are given, all user IDs with stale device lists are returned.
 func (d *mockDeviceListUpdaterDatabase) StaleDeviceLists(ctx context.Context, domains []gomatrixserverlib.ServerName) ([]string, error) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
 	var result []string
-	for userID := range d.staleUsers {
+	for userID, isStale := range d.staleUsers {
+		if !isStale {
+			continue
+		}
 		_, remoteServer, err := gomatrixserverlib.SplitID('@', userID)
 		if err != nil {
 			return nil, err
@@ -75,6 +81,8 @@ func (d *mockDeviceListUpdaterDatabase) StaleDeviceLists(ctx context.Context, do
 
 // MarkDeviceListStale sets the stale bit for this user to isStale.
 func (d *mockDeviceListUpdaterDatabase) MarkDeviceListStale(ctx context.Context, userID string, isStale bool) error {
+	d.mu.Lock()
+	defer d.mu.Unlock()
 	d.staleUsers[userID] = isStale
 	return nil
 }
@@ -247,3 +255,82 @@ func TestUpdateNoPrevID(t *testing.T) {
 	}
 
 }
+
+// Test that if we make N calls to ManualUpdate for the same user, we only do it once, assuming the
+// update is still ongoing.
+func TestDebounce(t *testing.T) {
+	db := &mockDeviceListUpdaterDatabase{
+		staleUsers: make(map[string]bool),
+		prevIDsExist: func(string, []int) bool {
+			return true
+		},
+	}
+	ap := &mockDeviceListUpdaterAPI{}
+	producer := &mockKeyChangeProducer{}
+	fedCh := make(chan *http.Response, 1)
+	srv := gomatrixserverlib.ServerName("example.com")
+	userID := "@alice:example.com"
+	keyJSON := `{"user_id":"` + userID + `","device_id":"JLAFKJWSCS","algorithms":["m.olm.v1.curve25519-aes-sha2","m.megolm.v1.aes-sha2"],"keys":{"curve25519:JLAFKJWSCS":"3C5BFWi2Y8MaVvjM8M22DBmh24PmgR0nPvJOIArzgyI","ed25519:JLAFKJWSCS":"lEuiRJBit0IG6nUf5pUzWTUEsRVVe/HJkoKuEww9ULI"},"signatures":{"` + userID + `":{"ed25519:JLAFKJWSCS":"dSO80A01XiigH3uBiDVx/EjzaoycHcjq9lfQX0uWsqxl2giMIiSPR8a4d291W1ihKJL/a+myXS367WT6NAIcBA"}}}`
+	incomingFedReq := make(chan struct{})
+	fedClient := newFedClient(func(req *http.Request) (*http.Response, error) {
+		if req.URL.Path != "/_matrix/federation/v1/user/devices/"+url.PathEscape(userID) {
+			return nil, fmt.Errorf("test: invalid path: %s", req.URL.Path)
+		}
+		close(incomingFedReq)
+		return <-fedCh, nil
+	})
+	updater := NewDeviceListUpdater(db, ap, producer, fedClient, 1)
+	if err := updater.Start(); err != nil {
+		t.Fatalf("failed to start updater: %s", err)
+	}
+
+	// hit this 5 times
+	var wg sync.WaitGroup
+	wg.Add(5)
+	for i := 0; i < 5; i++ {
+		go func() {
+			defer wg.Done()
+			if err := updater.ManualUpdate(context.Background(), srv, userID); err != nil {
+				t.Errorf("ManualUpdate: %s", err)
+			}
+		}()
+	}
+
+	// wait until the updater hits federation
+	select {
+	case <-incomingFedReq:
+	case <-time.After(time.Second):
+		t.Fatalf("timed out waiting for updater to hit federation")
+	}
+
+	// user should be marked as stale
+	if !db.staleUsers[userID] {
+		t.Errorf("user %s not marked as stale", userID)
+	}
+	// now send the response over federation
+	fedCh <- &http.Response{
+		StatusCode: 200,
+		Body: ioutil.NopCloser(strings.NewReader(`
+		{
+			"user_id": "` + userID + `",
+			"stream_id": 5,
+			"devices": [
+			  {
+				"device_id": "JLAFKJWSCS",
+				"keys": ` + keyJSON + `,
+				"device_display_name": "Mobile Phone"
+			  }
+			]
+		  }
+		`)),
+	}
+	close(fedCh)
+	// wait until all 5 ManualUpdates return. If we hit federation again we won't send a response
+	// and should panic with read on a closed channel
+	wg.Wait()
+
+	// user is no longer stale now
+	if db.staleUsers[userID] {
+		t.Errorf("user %s is marked as stale", userID)
+	}
+}