A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
What happens if due to in-game card effects I end up with two copies of a unique personality among the cards I own (say by having 2 Go us in play and both of them ending up with the Unique keyword)?
In other words does your deck becoming illegal during play effect you?
The legality of a deck is unaffected by changes of the game state of cards during the course of the game. Gaining the Unique keyword temporarily doesn't affect deck legality.