Have at least 2 players sitting behind the same router log in to the SAME server. Chances are they will kick each other out.
IMPORTANT NOTE: I have found a rudimentary fix for this. We have to turn off our modem and router for a few seconds so we get a new IP when we turn them back on (dynamic IP where we live). HOWEVER, this fixes the problem just for that session/server and we start kicking each other out again if we switch to another server (and we have to restart the router again, etc.).
I've made some research and a few people used to have the same problem in the mod so I'm surprised this hasn't been reported here so far.
This problem doesn't happen in any of the other online games that we play together by the way.