I think both IA_contacts and Photocontacts are thesame program but in different version.
If you want to disable one ,you can do it by delete it from \Windows\Startup.
Firstly ,this is an easy way to stop this problem.
In my suggestion, IA_contacts is older version and it cannot assign ringtone per contacts. If you need to use only default ringtone ,you should pick its.
But in Photocontacts which is newer version ,You can assign ringtone per contacts.
Secondly, in your Ext Rom, It may have Two version of CallerID.
To eliminate this problem, You need to edit your Ext ROM and remove one of this.
Now on WM6(Beta2)