Currently there seems to be a bug in the Windows Phone Store that causes previously purchased apps to not appear so on your account. As a result, if you try to re-download an app that you bought in the past, the Store will try to re-charge you for it. We were able to verify this on various phones of ours, someone with a different Microsoft account and it's even popping up in comments.
The glitch is sporadic too as sometimes we are able to get a previously purchased app (or game) with no issue. Ironically, Xbox games do not suffer from this anomaly, just everything else in the Store. We have heard that Microsoft is doing some backend changes and we bet this has something to do with that. As of now there is no service message from Redmond about this situation, so our advice is to wait it out as we hope it's very temporary. [Note: Do not attempt to re-purchase as you will be charged again]
Update: it appears to be regional as our George Ponder is not having problems.
If you're exhibiting this behavior, let us know in comments.