```ruby
class RemoveRedundantPipelineStages < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  def up
   redundant_stages_ids = <<~SQL
      SELECT id FROM ci_stages a WHERE (
        SELECT COUNT(*) FROM ci_stages b
          WHERE a.pipeline_id = b.pipeline_id AND a.name = b.name
      ) > 1
    SQL

    execute <<~SQL
      UPDATE ci_builds SET stage_id = NULL WHERE ci_builds.stage_id IN (#{redundant_stages_ids})
    SQL

    execute <<~SQL
      DELETE FROM ci_stages WHERE ci_stages.id IN (#{redundant_stages_ids})
    SQL
  end

  def down
    # noop
  end
end
```

Optimize SQL query that removes duplicated stages
user
```ruby
class RemoveRedundantPipelineStages < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  DOWNTIME = false

  def up
   redundant_stages_ids = <<~SQL
      SELECT id FROM ci_stages WHERE (pipeline_id, name) IN (
        SELECT pipeline_id, name FROM ci_stages
          GROUP BY pipeline_id, name HAVING COUNT(*) > 1
      )
    SQL

    execute <<~SQL
      UPDATE ci_builds SET stage_id = NULL WHERE ci_builds.stage_id IN (#{redundant_stages_ids})
    SQL

    execute <<~SQL
      DELETE FROM ci_stages WHERE ci_stages.id IN (#{redundant_stages_ids})
    SQL
  end

  def down
    # noop
  end
end
```
assistant