[migration] make this index creation conditional

db/migrate/20200806162649_add_index_to_statuses_reply.rb

@@ -2,6 +2,8 @@ class AddIndexToStatusesReply < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
   def change
-    add_index :statuses, :reply, algorithm: :concurrently
+    if !index_exists?(:statuses, :reply)
+      add_index :statuses, :reply, algorithm: :concurrently
+    end
   end
 end