If an user who belongs to an approval group and also is an individual in
the approval rule, `MergeService` will fail in the `after_merge`, which
will cause `MergeWorker` to retry again. Since the merge has been
successfully merged, `MergeWorker` will encounter an error since
`MergeService` is not idempotent.

This change fixes the issue by using the Array |= join method to add an
element to the Array, unless it is already present. This fixes the
immediate bug, but we will have to address the idempotency issues later.

Closes https://gitlab.com/gitlab-org/gitlab/issues/13488