While dual carrying weapons, use an item from inventory (like drink from a bottle) the weapon that was in your hands is seen on the ground but it seems it is in your hands as well, just invisible. Sometimes it will, after some time,reappear in my hands. Sometimes I have to log for the weapon to reappear in your hands.

While this happens you can't move anything into my hands.

If you swap my shouldered weapon with the weapon on the ground, inventory will show you carrying two guns on your back. Put one into your hands, it falls to the ground and exhibits the same issue as before.