More commonly heard in Discord and other similar platforms, repost in another server simply means two things:
1. The joke you just said was so funny that it should be in another server since it was funny.
2. Please stop talking in this server because we're busy, just repost in another server.
Usually it falls under the latter #2, as a sarcastic version of #1. Sorry.
Tom: Bro! What's up? You guys there?
Cassandra: Repost in another server.