diff --git a/account.go b/account.go
index 732ff77..8365275 100644
--- a/account.go
+++ b/account.go
@@ -418,10 +418,17 @@ func (a *Account) eventInternal(event *Event) error {
 			}
 		}
 
+		var cache_key string
 		if event.Id != "" {
-			cache_key := fmt.Sprintf("%s/event_seen/%s/%s",
+			// If the event has an ID, make sure it is processed only once
+			cache_key = fmt.Sprintf("%s/event_seen/%s/%s",
 				a.Protocol, mx_room_id, event.Id)
-			if !dbKvTestAndSet(cache_key, "yes") {
+			slot_key := dbKvSlotKey(cache_key)
+
+			dbLockSlot(slot_key)
+			defer dbUnlockSlot(slot_key)
+
+			if dbKvGet(cache_key) == "yes" {
 				// false: cache key was not modified, meaning we
 				// already saw the event
 				return nil
@@ -470,6 +477,12 @@ func (a *Account) eventInternal(event *Event) error {
 				}
 			}
 		}
+
+		// Mark message as received in db
+		if cache_key != "" {
+			dbKvPutLocked(cache_key, "yes")
+		}
+
 		return nil
 	}
 }
diff --git a/connector/connector.go b/connector/connector.go
index 38ce828..e8c382d 100644
--- a/connector/connector.go
+++ b/connector/connector.go
@@ -59,8 +59,14 @@ type Connector interface {
 	// Leave a channel
 	Leave(roomId RoomID)
 
-	// Send an event
-	Send(event *Event) error
+	// Send an event. Returns the ID of the created remote message.
+	// This ID is used to deduplicate messages: if it comes back, it should have the same Id
+	// than the one returned here.
+	// For backends that do not implement IDs (e.g. IRC), an empty string is returned.
+	// (FIXME how to deduplicate IRC messages?)
+	// The event that is fed in this function may have its ID already set,
+	// in which case the backend is free to re-use the ID or select a new one.
+	Send(event *Event) (string, error)
 
 	// Close the connection
 	Close()
diff --git a/connector/irc/irc.go b/connector/irc/irc.go
index dafe9db..4e5f4fd 100644
--- a/connector/irc/irc.go
+++ b/connector/irc/irc.go
@@ -198,9 +198,9 @@ func (irc *IRC) Leave(roomId RoomID) {
 	irc.conn.Cmd.Part(ch)
 }
 
-func (irc *IRC) Send(event *Event) error {
+func (irc *IRC) Send(event *Event) (string, error) {
 	if irc.conn == nil {
-		return fmt.Errorf("Not connected")
+		return "", fmt.Errorf("Not connected")
 	}
 
 	// Workaround girc bug
@@ -212,17 +212,17 @@ func (irc *IRC) Send(event *Event) error {
 	if event.Room != "" {
 		ch, err := irc.checkRoomId(event.Room)
 		if err != nil {
-			return err
+			return "", err
 		}
 		dest = ch
 	} else if event.Recipient != "" {
 		ui, err := irc.checkUserId(event.Recipient)
 		if err != nil {
-			return err
+			return "", err
 		}
 		dest = ui
 	} else {
-		return fmt.Errorf("Invalid target")
+		return "", fmt.Errorf("Invalid target")
 	}
 
 	if event.Attachments != nil && len(event.Attachments) > 0 {
@@ -230,7 +230,7 @@ func (irc *IRC) Send(event *Event) error {
 			url := at.URL()
 			if url == "" {
 				// TODO find a way to send them using some hosing of some kind
-				return fmt.Errorf("Attachment without URL sent to IRC")
+				return "", fmt.Errorf("Attachment without URL sent to IRC")
 			} else {
 				irc.conn.Cmd.Message(dest, fmt.Sprintf("%s (%s, %dkb)",
 					url, at.Mimetype(), at.Size()/1024))
@@ -243,9 +243,9 @@ func (irc *IRC) Send(event *Event) error {
 	} else if event.Type == EVENT_ACTION {
 		irc.conn.Cmd.Action(dest, event.Text)
 	} else {
-		return fmt.Errorf("Invalid event type")
+		return "", fmt.Errorf("Invalid event type")
 	}
-	return nil
+	return "", nil
 }
 
 func (irc *IRC) Close() {
diff --git a/connector/mattermost/mattermost.go b/connector/mattermost/mattermost.go
index 12ac604..52eb40f 100644
--- a/connector/mattermost/mattermost.go
+++ b/connector/mattermost/mattermost.go
@@ -243,7 +243,7 @@ func (mm *Mattermost) Leave(roomId RoomID) {
 	// Not supported? TODO
 }
 
-func (mm *Mattermost) Send(event *Event) error {
+func (mm *Mattermost) Send(event *Event) (string, error) {
 	post := &model.Post{
 		Message: event.Text,
 	}
@@ -254,29 +254,29 @@ func (mm *Mattermost) Send(event *Event) error {
 	if event.Room != "" {
 		ch, err := mm.checkRoomId(event.Room)
 		if err != nil {
-			return err
+			return "", err
 		}
 		post.ChannelId = ch
 	} else if event.Recipient != "" {
 		ui, err := mm.checkUserId(event.Recipient)
 		if err != nil {
-			return err
+			return "", err
 		}
 
 		_, resp := mm.conn.Client.CreateDirectChannel(mm.conn.User.Id, ui)
 		if resp.Error != nil {
-			return resp.Error
+			return "", resp.Error
 		}
 		channelName := model.GetDMNameFromIds(ui, mm.conn.User.Id)
 
 		err = mm.conn.UpdateChannels()
 		if err != nil {
-			return err
+			return "", err
 		}
 
 		post.ChannelId = mm.conn.GetChannelId(channelName, "")
 	} else {
-		return fmt.Errorf("Invalid target")
+		return "", fmt.Errorf("Invalid target")
 	}
 
 	if event.Attachments != nil {
@@ -284,28 +284,28 @@ func (mm *Mattermost) Send(event *Event) error {
 		for _, file := range event.Attachments {
 			rdr, err := file.Read()
 			if err != nil {
-				return err
+				return "", err
 			}
 			defer rdr.Close()
 			data, err := ioutil.ReadAll(rdr)
 			if err != nil {
-				return err
+				return "", err
 			}
 			up_file, err := mm.conn.UploadFile(data, post.ChannelId, file.Filename())
 			if err != nil {
 				log.Warnf("UploadFile error: %s", err)
-				return err
+				return "", err
 			}
 			post.FileIds = append(post.FileIds, up_file)
 		}
 	}
 
-	_, resp := mm.conn.Client.CreatePost(post)
+	created_post, resp := mm.conn.Client.CreatePost(post)
 	if resp.Error != nil {
 		log.Warnf("CreatePost error: %s", resp.Error)
-		return resp.Error
+		return "", resp.Error
 	}
-	return nil
+	return created_post.Id, nil
 }
 
 func (mm *Mattermost) Close() {
diff --git a/connector/xmpp/xmpp.go b/connector/xmpp/xmpp.go
index dcf1db6..efaaf64 100644
--- a/connector/xmpp/xmpp.go
+++ b/connector/xmpp/xmpp.go
@@ -306,13 +306,13 @@ func (xm *XMPP) Leave(roomId RoomID) {
 	xm.conn.LeaveMUC(string(roomId))
 }
 
-func (xm *XMPP) Send(event *Event) error {
+func (xm *XMPP) Send(event *Event) (string, error) {
 	if event.Attachments != nil && len(event.Attachments) > 0 {
 		for _, at := range event.Attachments {
 			url := at.URL()
 			if url == "" {
 				// TODO find a way to send them using some hosing of some kind
-				return fmt.Errorf("Attachment without URL sent to XMPP")
+				return "", fmt.Errorf("Attachment without URL sent to XMPP")
 			} else {
 				event.Text += fmt.Sprintf("\n%s (%s, %dkb)",
 					url, at.Mimetype(), at.Size()/1024)
@@ -320,6 +320,10 @@ func (xm *XMPP) Send(event *Event) error {
 		}
 	}
 
+	if event.Id == "" {
+		event.Id = xid.New().String()
+	}
+
 	log.Tracef("xm *XMPP Send %#v\n", event)
 	if len(event.Recipient) > 0 {
 		_, err := xm.conn.Send(gxmpp.Chat{
@@ -327,20 +331,17 @@ func (xm *XMPP) Send(event *Event) error {
 			Remote: string(event.Recipient),
 			Text:   event.Text,
 		})
-		return err
+		return event.Id, err
 	} else if len(event.Room) > 0 {
-		if event.Id == "" {
-			event.Id = xid.New().String()
-		}
 		_, err := xm.conn.Send(gxmpp.Chat{
 			Type:   "groupchat",
 			Remote: string(event.Room),
 			Text:   event.Text,
 			ID:     event.Id,
 		})
-		return err
+		return event.Id, err
 	} else {
-		return fmt.Errorf("Invalid event")
+		return "", fmt.Errorf("Invalid event")
 	}
 }
 
diff --git a/db.go b/db.go
index f9bed06..cdbdca1 100644
--- a/db.go
+++ b/db.go
@@ -163,6 +163,14 @@ func dbKvPut(key string, value string) {
 	dbLockSlot(slot_key)
 	defer dbUnlockSlot(slot_key)
 
+	dbKvPutLocked(key, value)
+}
+
+// Variant of dbKvPut that does not take a lock,
+// use this if the slot is already locked
+func dbKvPutLocked(key string, value string) {
+	slot_key := dbKvSlotKey(key)
+
 	var entry DbKv
 	db.Where(&DbKv{Key: key}).Assign(&DbKv{Value: value}).FirstOrCreate(&entry)
 	dbCache.Add(slot_key, value)
@@ -179,9 +187,7 @@ func dbKvTestAndSet(key string, value string) bool {
 		return false
 	}
 
-	var entry DbKv
-	db.Where(&DbKv{Key: key}).Assign(&DbKv{Value: value}).FirstOrCreate(&entry)
-	dbCache.Add(slot_key, value)
+	dbKvPutLocked(key, value)
 	return true
 }
 
diff --git a/server.go b/server.go
index 241a977..dc04006 100644
--- a/server.go
+++ b/server.go
@@ -145,18 +145,22 @@ func handleTxnEvent(e *mxlib.Event) error {
 			} else if e.Sender == pm_room.MxUserID {
 				ev.Author = acct.Conn.User()
 				ev.Recipient = pm_room.UserID
-				return acct.Conn.Send(ev)
+				_, err := acct.Conn.Send(ev)
+				return err
 			}
 		} else if room := dbIsPublicRoom(e.RoomId); room != nil {
-			cache_key := fmt.Sprintf("%s/event_seen/%s/%s",
-				room.Protocol, e.RoomId, ev.Id)
-			dbKvPut(cache_key, "yes")
 			// If this is a regular room
 			acct := FindJoinedAccount(e.Sender, room.Protocol, room.RoomID)
 			if acct != nil {
 				ev.Author = acct.Conn.User()
 				ev.Room = room.RoomID
-				return acct.Conn.Send(ev)
+				created_ev_id, err := acct.Conn.Send(ev)
+				if err == nil && created_ev_id != "" {
+					cache_key := fmt.Sprintf("%s/event_seen/%s/%s",
+						room.Protocol, e.RoomId, created_ev_id)
+					dbKvPut(cache_key, "yes")
+				}
+				return err
 			} else {
 				mx.RoomKick(e.RoomId, e.Sender, fmt.Sprintf("Not present in %s on %s, please talk with Easybridge to rejoin", room.RoomID, room.Protocol))
 				return fmt.Errorf("not joined %s on %s", room.RoomID, room.Protocol)