Actually, we do clean that data up with our corrections system. We have a large fingerprint database which allows us to work out when a track is the same and we update the charts across the site to reflect this. We even expose this as a service in the API:

http://www.last.fm/api/show?service=446 http://www.last.fm/api/show?service=447

What you're probably seeing there is tracks which are so new that we've not determined similar fingerprints for them yet, or those changes have not propagated to the charts (we use a lot of caching as you might imagine). In fact, checking the chart now it seems that that particular problem has cleared up.

Also, artist disambiguation is a _hard_ problem, but it's something that we'd very much like to address.