Depends on D19181. Ref T11015. This nukes duplicates from the table if they exist, then adds a unique key.

(Duplicates should not exist and can not be added with any recent version of the web UI.)

• Tried to add duplicates with web UI, didn't have any luck.
• Explicitly added duplicates with manual INSERTs.
• Viewed packages in web UI and saw duplicates.
• Ran migrations, got a clean purge and a nice unique key.
• There's still no way to actually hit a duplicate key error in the UI (unless you can collide hashes, I suppose), this is purely a correctness/robustness change.