diff --git a/syncapi/notifier/notifier.go b/syncapi/notifier/notifier.go
index 4e1a3c2a3..7c1e3c2db 100644
--- a/syncapi/notifier/notifier.go
+++ b/syncapi/notifier/notifier.go
@@ -22,6 +22,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
 	log "github.com/sirupsen/logrus"
 )
 
@@ -64,6 +65,9 @@ func NewNotifier() *Notifier {
 // SetCurrentPosition sets the current streaming positions.
 // This must be called directly after NewNotifier and initialising the streams.
 func (n *Notifier) SetCurrentPosition(currPos types.StreamingToken) {
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
+
 	n.currPos = currPos
 }
 
@@ -257,7 +261,7 @@ func (n *Notifier) SharedUsers(userID string) (sharedUsers []string) {
 			sharedUsers = append(sharedUsers, n.JoinedUsers(roomID)...)
 		}
 	}
-	return sharedUsers
+	return util.UniqueStrings(sharedUsers)
 }
 
 func (n *Notifier) IsSharedUser(userA, userB string) bool {
@@ -328,7 +332,7 @@ func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
 	// This is just the bulk form of addJoinedUser
 	for roomID, userIDs := range roomIDToUserIDs {
 		if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
-			n.roomIDToJoinedUsers[roomID] = make(userIDSet)
+			n.roomIDToJoinedUsers[roomID] = make(userIDSet, len(userIDs))
 		}
 		for _, userID := range userIDs {
 			n.roomIDToJoinedUsers[roomID].add(userID)
@@ -343,7 +347,7 @@ func (n *Notifier) setPeekingDevices(roomIDToPeekingDevices map[string][]types.P
 	// This is just the bulk form of addPeekingDevice
 	for roomID, peekingDevices := range roomIDToPeekingDevices {
 		if _, ok := n.roomIDToPeekingDevices[roomID]; !ok {
-			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet)
+			n.roomIDToPeekingDevices[roomID] = make(peekingDeviceSet, len(peekingDevices))
 		}
 		for _, peekingDevice := range peekingDevices {
 			n.roomIDToPeekingDevices[roomID].add(peekingDevice)
@@ -416,7 +420,7 @@ func (n *Notifier) fetchUserStreams(userID string) []*UserDeviceStream {
 	if !ok {
 		return []*UserDeviceStream{}
 	}
-	streams := []*UserDeviceStream{}
+	streams := make([]*UserDeviceStream, 0, len(user))
 	for _, stream := range user {
 		streams = append(streams, stream)
 	}
@@ -514,10 +518,10 @@ func (n *Notifier) removeEmptyUserStreams() {
 }
 
 // A string set, mainly existing for improving clarity of structs in this file.
-type userIDSet map[string]bool
+type userIDSet map[string]struct{}
 
 func (s userIDSet) add(str string) {
-	s[str] = true
+	s[str] = struct{}{}
 }
 
 func (s userIDSet) remove(str string) {
@@ -534,10 +538,10 @@ func (s userIDSet) values() (vals []string) {
 
 // A set of PeekingDevices, similar to userIDSet
 
-type peekingDeviceSet map[types.PeekingDevice]bool
+type peekingDeviceSet map[types.PeekingDevice]struct{}
 
 func (s peekingDeviceSet) add(d types.PeekingDevice) {
-	s[d] = true
+	s[d] = struct{}{}
 }
 
 // nolint:unused