The Ex Nihilo perk gives you a 20% chance to craft for free. If it procs while you are crafting an iconic weapon, the original weapon will not be expended to create the new weapon. As a result, you will have two copies of the iconic weapon. Both of these weapons can be crafted and upgraded further, and can be used and modded as normal. But neither of these weapons can be dropped, sold, or placed into your stash.