turn mentions into set

Author: RealCyGuy

core/thread.py

@@ -1061,7 +1061,7 @@ def get_notifications(self) -> str:
             self.bot.config["notification_squad"].pop(key)
             self.bot.loop.create_task(self.bot.config.update())
 
-        return " ".join(mentions)
+        return " ".join(set(mentions))
 
     async def set_title(self, title) -> None:
         user_id = match_user_id(self.channel.topic)