Downloading previous purchases in iTunes says "Buy"
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 As long as you didn't mistake a newer version of the song you can "buy" again for free.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.