Participant export with non main phone extension create unwanted duplicates

Create a contact with 3 phone numbers :

• one main phone number (home)
• 2 work phone number

Export participants (including the one created above) of an event with following mapping fields :

• display name
• phone work phone
• phone work extension

I expect to have 2 entry for this contact (one for each work phone but there is 4.

See in demo site (drupal7 - civi4.2) :

• contact = http://drupal.demo.civicrm.org/civicrm/contact/view?reset=1&cid=12
• export participant http://drupal.demo.civicrm.org/civicrm/event/search?reset=1&force=1&event=1&status=false with DuplicateExt field mapping