I was looking for a duplicate mp3 finder too and found this old thread and the recommendations do not even exist anymore. I researched further just to find that most of those duplicate finders claim to find duplicate songs and all they do is find exact duplicates.
If you want something that works try this tool:
Duplicate Song Finder
It compares the actual audio in the files and can find similar songs even if they are not marked with ID3 tags. There is a mode to find similar ID3 tags too.
The internal preview is very cool to as it continues to play from the same position while you switch between the duplicates letting you hear which file has better quality.