res_pjsip: Fix multiple of the same contact in "pjsip show contacts".

The code for gathering contacts could result in the same contact
being retrieved and added to the list multiple times. The container
which stores the contacts to display will now only allow a contact
to be added to it once instead of multiple times.

ASTERISK-28228

Change-Id: I805185cfcec03340f57d2b9e6cc43c49401812df