Q: itunes charged me for something I already bought?

I've never pre-ordered anything before so I'm a bit new to this.

Last Friday I pre-ordered an album. There were 2 songs that were already available that I had previously bought, and it showed "purchased" and also that I would get a discount for already having those songs (the album would only cost $6.01 instead of $7.99). But when I pre-ordered it went ahead and charged me for the songs that I already had. Why would it do this?! Is this normal or is there some kind of error?

Thanks