Mac/Windows/Linux: dupeGuru is a free, open source and cross-platform utility that will scan and help you remove duplicate audio files from your music library. The app uses filename, creation date, tag information and metadata, and even the audio fingerprint of the file to determine which songs are duplicates, even if they're named differently.
dupeGuru ME (Music Edition) supports most popular audio file formats, including MP3, OGG, and both traditional and lossless AAC and WMA (notably absent is FLAC, unfortunately.) Once installed, point the app at the folder where your music lives, and tell it to match dupes by filename, tags, file content or audio content (audio fingerprint). The app also supports complex search filters, and lets you add regular expressions or other logic to make sure you catch everything you're looking for.
When dupeGuru finds duplicates, it presents them to you to decide what to do with them. You can delete it immediately, move it elsewhere for further review, or even play the file right there to test it. If your music library is big enough that using your favourite player's "find duplicates" feature just isn't cutting it anymore, dupeGuru ME may be worth a look.