I've noticed that sometimes when I buy singles, the "Buy Album" price goes down, basically crediting me for the singles I've already purchased. But sometimes not. Sometimes when I want to buy the album, after buying a couple singles off the album, iTunes will just warn me that I'm about to have duplicates, and won't credit me with the songs I've already purchased. Basically, I have to buy those songs again if I click "Buy album."
What's the story? What are the circumstances in which iTunes will credit purchases, versus not crediting purchases? Is it a time issue (i.e., you only get a certain amount of time to buy the album after buying singles?)