Electronic delivery ain’t no record store

I bought a song and later an album off of the iTunes Music Store today and two problems came to mind:

1) I bought a single song while I was doing a zillion other things (other downloads, IM, ssh sessions on multiple servers, email checks) and there’s a two second bad chunk in the middle of the song that’s kind of like a CD skip but with more feedback. Maybe it was due to all the other network traffic, where do I exchange the downloaded song for a complete file? Can’t iTunes check the download against a hash and redownload it if there’s a mismatch?

2) I bought a “greatest hits” album from the same group and realized the single download I bought five minutes before is on it. How can I return my single song purchase? Could the iTMS given me a discount before I bought the same song twice?