Duplicate Primary Key Error with Percona Cluster

We recently setup our database with a Galera Cluster.
I have been getting the following errors every few hours:
Duplicate entry ‘XXXXXXX’ for key ‘PRIMARY’

This occurs with a number of tables, all using InnoDB.

We are using 3 machines in our cluster.
a. Master
b. Master
c. Backup - in case a SST is needed or the other two go down.

wsrep_auto_increment_control is ON.
innodb_autoinc_lock_mode is set to 2 (as the Percona documentation says this is mandatory).

What is causing it?
What can we do about it, without using only one Database for writes?

Thanks is advance.

I have the same issue with the same architecture.
Did you find the source of your problem or a workaround?

Can you show an example of such transaction together with table definition? And paste my.cnf. Do you use any MyISAM tables?