[Pacemaker] MySQL, Percona replication manager - split brain

[Andrew]

Hi all.

I use Percona as RA on cluster (nothing mission-critical, currently - 
just zabbix data); today after restarting MySQL resource (crm resource 
restart p_mysql) I've got a split brain state - MySQL for some reason 
started first at ex-slave node, ex-master starts later (possibly I've 
set too small timeout to shutdown - only 120s, but I'm not sure).

After restart resource on both nodes it seems like mysql replication was 
ok - but then after ~50min it fails in split brain again for unknown 
reason (no resource restart was noticed).

In 'show replication status' there is an error in table caused by unique 
index dup.

So I have a questions:
1) Which thing causes split brain, and how to avoid it in future?
2) How to resolve split brain state? Is it enough just to wait for 
failure, then - restart mysql by hand and clean row with dup index in 
slave db, and then run resource again? Or there is some automation for 
such cases?