Adds Slack error code skip retry

src/dispatch/plugins/dispatch_slack/incident/interactive.py

@@ -815,6 +815,10 @@ def handle_after_hours_message(
     participant = participant_service.get_by_incident_id_and_email(
         db_session=db_session, incident_id=context["subject"].id, email=user.email
     )
+    # handle no participant found
+    if not participant:
+        return
+
     # get their timezone from slack
     try:
         owner_tz = (dispatch_slack_service.get_user_info_by_email(client, email=owner_email))["tz"]

src/dispatch/plugins/dispatch_slack/service.py

@@ -104,7 +104,7 @@ def should_retry(exception: Exception) -> bool:
     match exception:
         case SlackApiError():
             # Don't retry for re-adding users in channel.
-            if exception.response["error"] == SlackAPIErrorCode.USER_IN_CHANNEL:
+            if exception.response["error"] in [SlackAPIErrorCode.USER_IN_CHANNEL, SlackAPIErrorCode.ALREADY_IN_CHANNEL]:
                 return False
             # Retry if it's not a fatal error
             return exception.response["error"] != SlackAPIErrorCode.FATAL_ERROR