celebration of cool iTunes feature turns to fear

So, I screwed up some files in my iTunes library. No prob. I grabbed them from my backup. I just dragged the whole album of MP3s into iTunes. The files got copied in and then I removed the duplicates. I had one AAC-encoded album that I did this to. For this album, iTunes only imported the tracks I didn’t have. At first, I thought that this was really cool. iTunes knows that I already have these tracks, so it didn’t bother. Then, I thought ITUNES KNOWS I HAVE THESE TRACKS. So, is there some sort of GUID in AAC? Is that GUID registered anywhere? If I encode my tracks as AAC and give them to you, can they be traced back to me? If this was some simple filename/filesize scheme, it would have caught the MP3 duplicates as well.