Bug overview description.
I stumbled upon a user who as favorited certain posts twice.
Using this search with a md5 filter returns the same file twice. If you look at the favs of the post the user is also listed twice there. This isn't the only occurence for this user, just the first one I found.

If you use the api to paginate the users favs it will be listed twice using this link

Also happens for (on the same user):
6bc9a816d8b2ad426514b87075fc6b82
4903d445946d5063e62f28fec8937800

What part(s) of the site page(s) are affected?
The search page as well as the posts page

What is the expected behavior?
The post should be returned once.

Can you reproduce the bug every time?
Yes (I don't actually know what to do to fav twice)

This is known and definitely not new. I'm not sure what causes it.

https://e621.net/forum/show/178562 <- old thread on the same issue

I tried a quick search but didn't find it. Well, maybe it'll get fixed with the new site

Shot in the dark: does this have to do with the transferal of favorites when a post is deleted for a superior version?
If someone favorited both versions before the transfer, would it double it?

I don't think so. Only one of the posts has a deleted child. Even though you can't see them on the website the api tells you the id of the inferior post.

Seems to have fixed itself while migrating the data to the new db schema.