Deduplicates participants lists before they're used for adding users …

…to conversations or threads (#3533)

* Deduplicates list of Slack user ids before using them for looping them into cases

* dedupes participants lists in plugin module

* removes blank line

src/dispatch/plugins/dispatch_slack/plugin.py

@@ -238,7 +238,7 @@ def send_ephemeral(
     def add(self, conversation_id: str, participants: List[str]):
         """Adds users to conversation."""
         client = create_slack_client(self.configuration)
-        participants = [resolve_user(client, p)["id"] for p in participants]
+        participants = [resolve_user(client, p)["id"] for p in set(participants)]
 
         archived = conversation_archived(client, conversation_id)
         if not archived:
@@ -247,7 +247,7 @@ def add(self, conversation_id: str, participants: List[str]):
     def add_to_thread(self, conversation_id: str, thread_id: str, participants: List[str]):
         """Adds users to a thread conversation."""
         client = create_slack_client(self.configuration)
-        participants = [resolve_user(client, p)["id"] for p in participants]
+        participants = [resolve_user(client, p)["id"] for p in set(participants)]
         add_users_to_conversation_thread(client, conversation_id, thread_id, participants)
 
     def archive(self, conversation_id: str):