If two contacts are participants of the same event, merged contact duplicates the events

When merging contacts, if the 'events' box is checked to move events over, if duplicate contact A had registered for the same event as duplicate contact B, then after the merge it shows that event twice on their contact page.

We should special-case events shared by both A and B and make sure the resulting contact is associated only once with them.

Note that the blind assumption that the participations are exactly the same might not be a good one; check PeterD's comments at http://forum.civicrm.org/?topic=11796