backport-resolve-issue: prune duplicate URLs

backport-resolve-issue: prune duplicate URLs

I have seen PR descriptions with a single backport tracker URL mentioned twice.
When backport-resolve-issue hit one of these, it would say:

Found backport tracker: https://tracker.ceph.com/issues/39272
Found backport tracker: https://tracker.ceph.com/issues/39272
-----------------------------------------------------------------
INFO:root:Tracker https://tracker.ceph.com/issues/39272 links to PR https://github.com/ceph/ceph/pull/29153
INFO:root:Backport Tracker 39272 target version already populated with correct value v12.2.13
INFO:root:Backport Tracker 39272 status is already set to Resolved
-----------------------------------------------------------------
INFO:root:Tracker https://tracker.ceph.com/issues/39272 links to PR https://github.com/ceph/ceph/pull/29153
INFO:root:Backport Tracker 39272 target version already populated with correct value v12.2.13
INFO:root:Backport Tracker 39272 status is already set to Resolved
=================================================================

This commit fixes the issue.

Signed-off-by: Nathan Cutler <ncutler@suse.com>