Fix error when duplicate users are merged in approvers list
If a user who belongs to an approval group and also is an individual in
the approval rule,
MergeService will fail in the
MergeWorker to retry again. Since the merge has been
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 #13488 (closed)