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 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 #13488 (closed)

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• 07730b0e - Fix error when duplicate users are list in approvers list

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changed milestone to %12.3

added 1 deleted label

added devopscreate priority2 typebug labels

changed the description

assigned to @tkuah and unassigned @stanhu

mentioned in issue #13488 (closed)

added severity2 label

While this is labeled as a P2 in the issue, I think it warrants a P1 given that:

1. It's happening on many gitlab-org/gitlab merge requests
2. It's causing many duplicate MergeWorkers to run
3. It's causing a LOT of confusion whether a merge request has been merged or not.

added Pick into auto-deploy label

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Category	Reviewer	Maintainer
backend	Oswaldo Ferreira (@oswaldo)	Grzegorz Bizon (@grzesiek)

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changed title from Fix error when duplicate users are list in approvers list to Fix error when duplicate users are merged in approvers list

mentioned in issue #32185 (closed)

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• 6e4a4465 - Fix error when duplicate users are merged in approvers list

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approved this merge request

merged

mentioned in commit 404eee31

Thanks for digging into @stanhu

In terms of idempotency, is there anything specific we can open an issue about ? My understanding is that different merge strategies will potentially have different issues /cc @dosuken123

Automatically picked into 12-3-auto-deploy-20190922, will merge into 12-3-auto-deploy-20190922 ready for 12.3.0-ee.