Don't reopen duplicate incidents

What does this MR do and why?

In order to reduce the noise in Slack, we should never reopen incidents that were closed as duplicates because the investigation should happen in the canonical opened incidents anyway, so all the activity in duplicated incidents (and Slack notifications) is just noise.

This was brought up in yesterday's team meeting as well as in Slack today: https://gitlab.slack.com/archives/C04RC5QTGBT/p1717685072214789 (internal).

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-policies-with-a-dry-run on how to perform dry-runs for new policies.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/blob/master/doc/reactive/best_practices.md#use-the-sandbox-to-test-new-processors on how to make sure a new processor can be tested.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review

Closes #1542 (closed)