No matter which hash function you use, you will _not_ get 100% unique identifiers, because you _can't_!
Just do the freaking math, and you'll see for yourself - it's quite obvious, actually!
Yeah, I was confused. I'm a product guy, so math isn't my strongest point. It looks like combining Md5 + size + Sha1 should be enough to get a almost unique file id. Thank you all for the replies.