A single person cheating 2 accounts in same game.
Cheaters who play in one game with multiple accounts to gain an advantage are no more than 2% of all players. However these couple of percentage points ruin the game for the other ~98% of players.
Consider ~equivalent of a Steam VAC type system, where community bans the few trolls, and improves the experience for all non-cheating users.
In 2-3 times its happened to me - its not same guy with same name - its same guy with different name - e.g. playing under two different ips/machines. Katan algo does not know this (afaik) and does not allow for karma free exit
telltale signs this is happening:
1. Potentially same IP / same /browser/OS (circumventable by VPN, using 2 different connections - ISP + cellphone etc)
2. lightning fast trades only between player A account 1 player A account 2 => giving player A an unfair resource advantage (cheating)
3. Account 1 + 2 have equal number of games played / exited.