Only add mentions to the mentions timeline once.

src/modules/statuses.js

@@ -138,10 +138,13 @@ const addNewStatuses = (state, { statuses, showImmediately = false, timeline, us
       if (statusType(status) === 'status' && find(status.attentions, { id: user.id })) {
         const mentions = state.timelines.mentions
 
+        // Add the mention to the mentions timeline
+        if (timelineObject !== mentions) {
           mergeOrAdd(mentions.statuses, status)
           mentions.newStatusCount += 1
 
           sortTimeline(mentions)
+        }
 
         addNotification({ type: 'mention', status, action: status })
       }