We are seeing some errors when migrating and adding indexes on `answer_post_id`.
```
#<StandardError:"An error has occurred, all later migrations canceled:\n\nPG::UniqueViolation: ERROR: could not create unique index \"index_discourse_solved_solved_topics_on_answer_post_id\"\nDETAIL: Key (answer_post_id)=(13006) is duplicated.\n">
```
This PR modifies the earlier migration, and also adds one before the addition of indexes to remove duplicates.
