What happened with Create a Similar Playlist?

[waziot]

When this feature was first introduced it was great, but now whenever I use it there are doubles of the same song in the playlist as well as songs from the original playlist I had "created similar" from. 

I feel like this is an easy fix to create hard and fast rules that two of the same song can't be generated in the same playlist and songs from the previous playlist can't be pushed through? This has been an ongoing problem for at least a year now.

Why is this still an issue?