Check for duplication before creating incident

What does this MR do and why?

Bug fix for Deduplicate or auto-close as duplicates master-... (gitlab-org/gitlab#395280 - closed). The bug was initially reported in !2295 (comment 1584171170).

The "mark as duplicate" automation has not really been working due to a bug from the first iteration. In short, we should check for duplication BEFORE the new incident is created. Otherwise, PipelineIncidentFinder will always return the one we have just created and try to compare with itself for duplication. This one obviously will not have been marked as a duplicate since it was just created.

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