I was mistakenly sent an item I'd returned, what to do? Keep it?

I bought an expensive item from the website of one of the big box hardware stores -- impulse buy. It was shipped and delivered promptly and I returned it to the local brick and mortar as soon as I received it. They accepted the return and refunded my card promptly.

Fast forward a week, the item shows back up on my doorstep (re-delivered by Fedex). I've spent 30+ minutes and called multiple times, trying to reach a rep at the store I returned the item. I'm invariably eventually disconnected.

The item is large and heavy -- over 100lbs. It took me a whole afternoon to load it up, schlep to the nearest brick and mortar, park, wait in the return line, etc.

So.. what should I do here?