Yeah that was what happened last time. I tried purchasing something I already had and it did give me a message saying that I already had it. This time though, no warning, 400 points deducted and nothing new. I filed a ticket with CS before I made this post and so far, all I've got is a reply that the ticket is being sent higher up the chain.
What gets me is that if this was addressed and the Market actually displayed in some way that we'd already purchased something previously, I would be more likely to spend more on it. I'm one of those people who will make sure they have every possible costume option available. What I'm not is someone who enjoys having a spreadsheet so they can keep track of what they've purchased so they don't re-buy stuff.