1. Start game as rifleman (has both uniform and vest)
2. Open inventory.
3. Remove all items from the vest and uniform.
4. Drag uniform to the ground slot.
5. Drag uniform on top of vest slot.

Expected:
Vest slot should be "red" indicating uniform cannot be dropped. Dropping the uniform should cancel to drag action.

Actual:
Vest slot is highlighted as an valid drop target. Dropping the uniform on the vest slot results in a vest item being added to the ground slot (i.e. duped). Uniform remains on the ground, and original vest remains worn.

If uniform contains a single item the drop action is prevented as expected. Vest does not have to be empty, just contain enough room for the uniform to fit.

It appears the reverse, i.e. dropping the vest on the uniform slot, does not cause this bug to appear.