Allow removal of issues marked as duplicate

Summary

When an issue is marked as a duplicate of another via /duplicate quick action, there's no way to unlink them. Even if it's re-added as a Related issue, it doesn't seem to lose its "duplicate" relationship, so that when the original issue is closed, it is always considered to be Closed (Duplicated). This is also more problematic when you mistakenly added the duplicate association but you can never remove it.

Steps to reproduce

  1. Create issue A
  2. Add another issue B, and in A - you accidentally mark B as a duplicate: /duplicate B
  3. Close A
  4. It always shows up as Closed (Duplicate)

Working example in an Internal Slack discussion in #s_plan: https://gitlab.slack.com/archives/C72HPNV97/p1596051855317900

Example Project

Noted behaviour in: #15356 (comment 387814369)

What is the current bug behavior?

Can never remove the "duplicated" association

What is the expected correct behavior?

Provide some way to remove the duplicate, e.g. /remove_duplicate quick action

Relevant logs and/or screenshots

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Output of checks

Happens on GitLab.com

Results of GitLab environment info

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Results of GitLab application Check

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sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)


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Possible fixes

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Edited by 🤖 GitLab Bot 🤖