ICloud – Sync iCloud TV show purchases between AppleTV and iTunes

If you really purchased it, it will show buy and then tell you it's free since you already bought it. As long as you didn't mistake a newer version of the song you can "buy" again for free. There is a report a problem link if you do accidentally re-buy substantially the same item and I wouldn't hesitate to ask for a refund when the storekeeper doesn't make it easier to know if it's a re-download or a buy.

In any of the common scenarios in which you store your Store credentials on devices under the control of others, it makes good security sense to have those be different from the ones you use for iCloud.

For example, it is common to share music among family members by using an Apple ID for purchases on all of the family's machines. If that Apple ID were the same as the one you used for iCloud, then (unless you can be sure that all family members have secured their machines) anyone who got his hands on one of those machines could potentially gain access to your e-mail, calendar, and documents (and, in typical configurations, could even log on to your machine if he had access to it).