diff --git a/syncapi/streams/stream_sendtodevice.go b/syncapi/streams/stream_sendtodevice.go
index 6a18df50..31c6187c 100644
--- a/syncapi/streams/stream_sendtodevice.go
+++ b/syncapi/streams/stream_sendtodevice.go
@@ -39,21 +39,13 @@ func (p *SendToDeviceStreamProvider) IncrementalSync(
 		return from
 	}
 
-	if len(events) > 0 {
-		// Clean up old send-to-device messages from before this stream position.
-		if err := p.DB.CleanSendToDeviceUpdates(req.Context, req.Device.UserID, req.Device.ID, from); err != nil {
-			req.Log.WithError(err).Error("p.DB.CleanSendToDeviceUpdates failed")
-			return from
-		}
-
-		// Add the updates into the sync response.
-		for _, event := range events {
-			// skip ignored user events
-			if _, ok := req.IgnoredUsers.List[event.Sender]; ok {
-				continue
-			}
-			req.Response.ToDevice.Events = append(req.Response.ToDevice.Events, event.SendToDeviceEvent)
+	// Add the updates into the sync response.
+	for _, event := range events {
+		// skip ignored user events
+		if _, ok := req.IgnoredUsers.List[event.Sender]; ok {
+			continue
 		}
+		req.Response.ToDevice.Events = append(req.Response.ToDevice.Events, event.SendToDeviceEvent)
 	}
 
 	return lastPos
diff --git a/syncapi/sync/requestpool.go b/syncapi/sync/requestpool.go
index b6b4779a..d908a962 100644
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@@ -25,6 +25,11 @@ import (
 	"sync"
 	"time"
 
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/sirupsen/logrus"
+
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	keyapi "github.com/matrix-org/dendrite/keyserver/api"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
@@ -35,10 +40,6 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/streams"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
-	"github.com/matrix-org/gomatrixserverlib"
-	"github.com/matrix-org/util"
-	"github.com/prometheus/client_golang/prometheus"
-	"github.com/sirupsen/logrus"
 )
 
 // RequestPool manages HTTP long-poll connections for /sync
@@ -251,6 +252,12 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *userapi.
 	waitingSyncRequests.Inc()
 	defer waitingSyncRequests.Dec()
 
+	// Clean up old send-to-device messages from before this stream position.
+	// This is needed to avoid sending the same message multiple times
+	if err = rp.db.CleanSendToDeviceUpdates(syncReq.Context, syncReq.Device.UserID, syncReq.Device.ID, syncReq.Since.SendToDevicePosition); err != nil {
+		syncReq.Log.WithError(err).Error("p.DB.CleanSendToDeviceUpdates failed")
+	}
+
 	// loop until we get some data
 	for {
 		startTime := time.Now()
diff --git a/syncapi/types/types.go b/syncapi/types/types.go
index 159fa08b..39b085d9 100644
--- a/syncapi/types/types.go
+++ b/syncapi/types/types.go
@@ -21,9 +21,10 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/tidwall/gjson"
+
+	"github.com/matrix-org/dendrite/roomserver/api"
 )
 
 var (
@@ -330,23 +331,23 @@ type Response struct {
 	NextBatch   StreamingToken `json:"next_batch"`
 	AccountData struct {
 		Events []gomatrixserverlib.ClientEvent `json:"events,omitempty"`
-	} `json:"account_data"`
+	} `json:"account_data,omitempty"`
 	Presence struct {
 		Events []gomatrixserverlib.ClientEvent `json:"events,omitempty"`
-	} `json:"presence"`
+	} `json:"presence,omitempty"`
 	Rooms struct {
-		Join   map[string]JoinResponse   `json:"join"`
-		Peek   map[string]JoinResponse   `json:"peek"`
-		Invite map[string]InviteResponse `json:"invite"`
-		Leave  map[string]LeaveResponse  `json:"leave"`
-	} `json:"rooms"`
+		Join   map[string]JoinResponse   `json:"join,omitempty"`
+		Peek   map[string]JoinResponse   `json:"peek,omitempty"`
+		Invite map[string]InviteResponse `json:"invite,omitempty"`
+		Leave  map[string]LeaveResponse  `json:"leave,omitempty"`
+	} `json:"rooms,omitempty"`
 	ToDevice struct {
-		Events []gomatrixserverlib.SendToDeviceEvent `json:"events"`
-	} `json:"to_device"`
+		Events []gomatrixserverlib.SendToDeviceEvent `json:"events,omitempty"`
+	} `json:"to_device,omitempty"`
 	DeviceLists struct {
 		Changed []string `json:"changed,omitempty"`
 		Left    []string `json:"left,omitempty"`
-	} `json:"device_lists"`
+	} `json:"device_lists,omitempty"`
 	DeviceListsOTKCount map[string]int `json:"device_one_time_keys_count,omitempty"`
 }