Remove index by name on ci_pipelines

changelogs/unreleased/cat-duplicate-ci-pipelines-index-215790.yml

+---
+title: Fix duplicate index removal on ci_pipelines.project_id
+merge_request: 31043
+author:
+type: fixed

db/migrate/20191229140154_drop_index_ci_pipelines_on_project_id.rb

@@ -8,10 +8,13 @@ class DropIndexCiPipelinesOnProjectId < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
   def up
-    remove_concurrent_index :ci_pipelines, :project_id
+    remove_concurrent_index_by_name :ci_pipelines, 'index_ci_pipelines_on_project_id'
+
+    # extra (duplicate) index that already existed on some installs
+    remove_concurrent_index_by_name :ci_pipelines, 'ci_pipelines_project_id_idx'
   end
 
   def down
-    add_concurrent_index :ci_pipelines, :project_id
+    add_concurrent_index :ci_pipelines, :project_id, name: 'index_ci_pipelines_on_project_id'
   end
 end