iZakaroN's SaveEditorTo copy an item; quick save the game, then drag an item out of your inventory, but not somewhere there is a red X. Without dropping first, quick load the game, then release the mouse button when it starts to load. This will produce a copy of the item on the ground, with the original still in your inventory.
There is a much easier method for items which can be stacked; drag a stack out of (or into) your inventory (or container) and hold the control key down before dropping. This will bring up the split stack pane; enter a negative number and hit enter. This can also be done in the trade window.
