Add a simple duplication prevention mechanism

On conduwuit at least, the sync() calls may sometimes trigger duplicate events, resulting in duplcated commands. This simple addition just records which event IDs are seen, adds them to a dequeue, and refuses to process duplicated ones.

src/niobot/client.py

@@ -1,4 +1,5 @@
 import asyncio
+import collections
 import functools
 import getpass
 import importlib
@@ -265,6 +266,8 @@ def __init__(
         self._sync_full_state = sync_full_state
         self.default_parse_mentions = default_parse_mentions
 
+        self._event_id_cache = collections.deque(maxlen=1000)
+
     @property
     def supported_server_versions(self) -> typing.List[typing.Tuple[int, int, int]]:
         """
@@ -407,9 +410,12 @@ async def update_read_receipts(self, room: U[str, nio.MatrixRoom], event: nio.Ev
 
     async def process_message(self, room: nio.MatrixRoom, event: nio.RoomMessage) -> None:
         """Processes a message and runs the command it is trying to invoke if any."""
+        if event.event_id in self._event_id_cache:
+            self.log.warning("Not processing duplicate message event %r.", event.event_id)
+            return
         if self.start_time is None:
             raise RuntimeError("Bot has not started yet!")
-
+        self._event_id_cache.append(event.event_id)
         self.message_cache.append((room, event))
         self.dispatch("message", room, event)
         if not isinstance(event, nio.RoomMessageText):