The first time i could duplicate a laser rifle in the middle of a battle in this way, other attempts to reproduce it have resulted in crashes.
To reproduce:
* drop soldier weapon (or bring an unarmed soldier)
* select another (armed) soldier and open his inventory
* click his weapon and drag it
* rotate the mouse wheel to select the first unarmed soldier without releasing LMB nor closing the inventory screen
* try to drop the weapon into the unarmed soldier's hand
* crash or duplication