DS turns 2 in April and already we've dealt with this scenario several times. It's always MIL and she always knew that the item in question had already been bought.

When we were setting up the nursery, my mom had told her that she had bought an aquarium for the crib. So she went out and bought the same one and gave it to us first. My mom returned hers. Next, she asked if I had a nursing pillow, I said yes (had just bought my breast friend). She went out and bought me the Mumbo, but then said she couldn't return it because she bought it at Target, which was closing down in Canada. I returned mine, and regretted it, because I hated the one she had bought.

Most recently, she asked what we were buying DS for Christmas, and stupidly, we told her. She went out and bought a couple of the same items (but different, cheaper brands). I think I handled it with grace, even though this behaviour really bothers me. I said "well, that's great, now he had x, y, z for your house!"

How would you have handled this situation? Once or twice, I'd say it's bad luck, but it's every single occasion, so it's not by chance, at all.