During an
IRC split (when two servers on the same
IRC network are
desyncronized), it's sometimes possible for two users to join the IRC network with the same nick, by logging in to disconnected servers. Since a server can't tell what nicks are occupied on the other server, the situation will persist for some time -- but once the servers reestablish connection, the matter would need to be solved.
The solution of a typical IRC server to this dilemma would be to disconnect both users sharing a nick (by issuing a /kill command).
The technique was abused by malicious users to disconnect other legitimate users and bots from IRC.