- Open up the housing vendor
- Choose the item you want 2x of
- Right click in the game world
- Purchase the item
It'll place one in the game world, and a second copy in your crate.
Edit: Doesn't 100% repro. Seems to be related to hitting "buy" vs "purchase", and some timing issue when closing the window. (normally, the item is removed from the game world when hitting purchase - the dupe happens when that bugs and doesn't)