I have developed game, and I have Coins, which users can buy from the Activity.
Problem was, that when User purchases one, they can't purchase these more, because it shows error "Item Already owned".
When user wants same amount coins with same price second time, and now this is not possible.
(I am using Google Play Billing)
You need to consume the purchase, so the item returns to a state where it can be purchased again. See this question.