Code review comment for lp:~renatofilho/address-book-service/fix-1269561

> Needs fixing:
> 1) create a contact with home email a@a.com
> 2) save the contact
> 3) edit the contact and add another home email a@a.com
> this fails

This bug already exists on the current version, and to avoid this MR grow even more, I created a different MR only for this bug:

https://code.launchpad.net/~renatofilho/address-book-service/fix-duplicated-fields/+merge/208040