Every single email should have a unique
Message-Id: header field.
The email announcing the opening and closure of an issue uses the same msg id. E.g., this is the msg id for the opening and closing of bug 16 of the cloudflare-tor repo:
Mail tools rely on unique message IDs for a variety of functions such as detecting duplicate messages. A common Procmail recipe deletes dupes by comparing the message ID to a cache of seen message IDs. Mutt signals that a message is a dupe with an “=” symbol, and is falsely flagging the closure messages as dupes.