It turns out that your canine companion in Fallout 4 is not only great at fetching weapons and mauling raiders to death, but also at magically multiplying your supplies - whether they're stacks of ammo, or books that improve your SPECIAL attributes. Her's how to do it: Drop the item you want to duplicate on the ground near Dogmeat, then command him to fetch it. When he stops to pick up the item, pick it up just before his head goes down to pick it up. If you time it right, Dogmeat will lean down to pick up the item after you've already picked it up, but magically duplicate the item and drop it at your feet (if you did this with a stack of ammo, he'll only bring you one bullet rather than the whole stack). To turn this single bullet into a stack of the same size as you originally dropped, drop your ammo stack on the ground alongside the one Dogmeat leaves for you. Go into Settings, Quicksave your game, then quit to the main menu. Return to the game, and you'll see two ammo stacks on the ground in front of you, both containing the same amount of ammo as your original stack.