A very simple but confusing puzzle.
A lady buys goods worth $ 20 from a shop. (shopkeeper selling the goods with zero profit).
The lady gives him 100 $ note. The shopkeeper gets the change from the next shop and keeps 20 for himself and returns $ 80 to d lady.
Later the shopkeeper of the next shop comes with the 100 $ saying “duplicate” and takes his money back.
How much LOSS did the shopkeeper face ?