I doubt that CRC checksums would really fix the problem when it comes to mp3s. There could be five identical versions of a song, but if there is id3 tag information in each song, it has to be identical in each song. Different id3 tags will add a different number of bytes to each file, changing the file size, so you can't rely on that, either.

For relatively small files, such as mp3s, there is little that can be done, however given that the typical file size IS so small, one can download it fairly quickly without difficulty.

For larger files (e.g. movies, divx, etc.) there are likely to be LESS original varsions floating around. Any bonehead can rip a cd track and compress it to an mp3; there's no real magic to it. But, I don't know many people who can rip a dvd, crunch it with divx, and share it.