Why does iTunes charge for songs already purchased?

When I buy a song from a single-song album, the song is bought but the album is not. If I tried to buy the album, it said that I've already purchased the song and asked me if I want to buy the song again. If I buy the song again, does iTunes charge me twice? Wouldn't purchasing a song from a single-song album automatically buy the album. This is very confusing!!

[Re-Titled by Moderator]