Duplicate registrations for the same role unless the role is default.

Joomla 2.5.*
CiviCRM 4.2.* (up to 4.2.7 at time of forum post)

ISSUE
Only the default role set in the event management configuration will recognise duplicate participants.

RECREATE (I have recreated in http://joomla.demo.civicrm.org)
1. Add 'role select' to a profile
2. Add role profile to event configuration
3. Register multiple times with a role other than the default set in the event management.
4. Register with the default role
5. Now try and register again with any role and duplicate will be recognised.

THE FIX (for 4.2.7)

/CRM/Event/Form/Registration/Register.php

LINE: 1457

FIND
$participant->role_id = $self->_values['event']['default_role_id'];

REPLACE WITH
$participant->role_id = $self->_values['event']['role_id'];