Is it possible for unique key to break master-master replication in mysql?
Though I've not tried this myself, I've done a quick documentation hunt and found that this part of the MySQL documentation is helpful.

The relevant part is:


  If a statement produces different errors on the master and the slave, the slave SQL thread terminates, and the slave writes a message to its error log and waits for the database administrator to decide what to do about the error. This includes the case that a statement produces an error on the master or the slave, but not both.


The next question would then be: "How do I know there has been an error?", to which I've found a useful script here, which could be run frequently as a cron task.