replication fails after upgrade to 5.6

[Mike Franon]

So I successfully upgraded a test db server from 5.0.96 all the way up to 5.6

Replication as the slave, where the master is 5.0.96, started working
for about 10 minutes and then got the following error:

[ERROR] Slave SQL: Error 'Duplicate entry 'data' for key 'PRIMARY'' on
query. Default database: 'test'. Query: 'UPDATE IGNORE , Error_code:
1062


All of our other slaves on 5.0.96 are fine, so I know it has to do
with 5.6 but just not sure what, when ir an mysql_upgrade everything
was OK

Thanks