fix(gateway): add message deduplication to Discord and Slack adapters

gateway/platforms/discord.py

@@ -449,6 +449,11 @@ def __init__(self, config: PlatformConfig):
         self._bot_task: Optional[asyncio.Task] = None
         # Cap to prevent unbounded growth (Discord threads get archived).
         self._MAX_TRACKED_THREADS = 500
+        # Dedup cache: message_id → timestamp.  Prevents duplicate bot
+        # responses when Discord RESUME replays events after reconnects.
+        self._seen_messages: Dict[str, float] = {}
+        self._SEEN_TTL = 300   # 5 minutes
+        self._SEEN_MAX = 2000  # prune threshold
 
     async def connect(self) -> bool:
         """Connect to Discord and start receiving events."""
@@ -539,6 +544,19 @@ async def on_ready():
 
             @self._client.event
             async def on_message(message: DiscordMessage):
+                # Dedup: Discord RESUME replays events after reconnects (#4777)
+                msg_id = str(message.id)
+                now = time.time()
+                if msg_id in adapter_self._seen_messages:
+                    return
+                adapter_self._seen_messages[msg_id] = now
+                if len(adapter_self._seen_messages) > adapter_self._SEEN_MAX:
+                    cutoff = now - adapter_self._SEEN_TTL
+                    adapter_self._seen_messages = {
+                        k: v for k, v in adapter_self._seen_messages.items()
+                        if v > cutoff
+                    }
+
                 # Always ignore our own messages
                 if message.author == self._client.user:
                     return

gateway/platforms/slack.py

@@ -13,6 +13,7 @@
 import logging
 import os
 import re
+import time
 from typing import Dict, Optional, Any
 
 try:
@@ -78,6 +79,11 @@ def __init__(self, config: PlatformConfig):
         self._team_clients: Dict[str, AsyncWebClient] = {}   # team_id → WebClient
         self._team_bot_user_ids: Dict[str, str] = {}          # team_id → bot_user_id
         self._channel_team: Dict[str, str] = {}                # channel_id → team_id
+        # Dedup cache: event_ts → timestamp.  Prevents duplicate bot
+        # responses when Socket Mode reconnects redeliver events.
+        self._seen_messages: Dict[str, float] = {}
+        self._SEEN_TTL = 300   # 5 minutes
+        self._SEEN_MAX = 2000  # prune threshold
 
     async def connect(self) -> bool:
         """Connect to Slack via Socket Mode."""
@@ -710,6 +716,20 @@ async def get_chat_info(self, chat_id: str) -> Dict[str, Any]:
 
     async def _handle_slack_message(self, event: dict) -> None:
         """Handle an incoming Slack message event."""
+        # Dedup: Slack Socket Mode can redeliver events after reconnects (#4777)
+        event_ts = event.get("ts", "")
+        if event_ts:
+            now = time.time()
+            if event_ts in self._seen_messages:
+                return
+            self._seen_messages[event_ts] = now
+            if len(self._seen_messages) > self._SEEN_MAX:
+                cutoff = now - self._SEEN_TTL
+                self._seen_messages = {
+                    k: v for k, v in self._seen_messages.items()
+                    if v > cutoff
+                }
+
         # Ignore bot messages (including our own)
         if event.get("bot_id") or event.get("subtype") == "bot_message":
             return