If you have the 1911 pistol with the 7rnd magazine in your hands, then remove the magazine to reload it, when you re-attached the magazine it leaves behind a copy of the magazine in your inventory. This copy magazine can't be used as a 2nd magazine so it's of no use, it just takes up a space in your inventory. You can't then throw the copy on the ground, you need to place the pistol on the ground, then the copy magazine, then pick the pistol back up.