Contact update gives backtrace when we update cms user contact with any other cms user email from contact form.

Steps to replicate :
1. Create two contacts , both should have associated cms user records.
2. Update email of one contact with email of another contact from contact form.

As we do have two unique indexes in civicrm_uf_match table :
1. contact_id and domain_id
2. uf_name and domain_id

It looks like during contact update we are checking only first unique index in CRM_Core_BAO_UFMatch::updateUFName() ;

Work around would be : We should check for both unique indexes.