ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "index_vulnerability_...

https://sentry.gitlab.net/gitlab/gitlabcom/issues/3165625/?referrer=gitlab_plugin

PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_vulnerability_occurrences_on_unique_keys"
DETAIL:  Key (project_id, primary_identifier_id, location_fingerprint, scanner_id)=(15642544, 1083447, \x27e5fbd2c567177e351b0b77385e48a275e66f26, 8489) already exists.

  lib/gitlab/database/load_balancing/connection_proxy.rb:126:in `block in write_using_load_balancer'
    connection.send(...)
  lib/gitlab/database/load_balancing/load_balancer.rb:112:in `block in read_write'
    yield connection
  lib/gitlab/database/load_balancing/load_balancer.rb:179:in `retry_with_backoff'
    return yield
  lib/gitlab/database/load_balancing/load_balancer.rb:110:in `read_write'
    retry_with_backoff do
  lib/gitlab/database/load_balancing/connection_proxy.rb:125:in `write_using_load_balancer'
    @load_balancer.read_write do |connection|
...
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ActiveRecord::RecordNotUnique: PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_vulnerability_occurrences_on_unique_keys"
DETAIL:  Key (project_id, primary_identifier_id, location_fingerprint, scanner_id)=(15642544, 1083447, \x27e5fbd2c567177e351b0b77385e48a275e66f26, 8489) already exists.

Implementation plan:

• Write a migration to remove unique index index_vulnerability_occurrences_on_unique_keys