You can add it to a wish list!
Simply find the app and instead of clicking the â€œBUYâ€ button, click the drop-down arrow next to it, and choose â€œAdd to Wish Listâ€. If you have already purchased the app (or any iTunes media for that fact), you will be told that you have already purchased the item.
Neat. Unfortunately you can only do this trick on a desktop computer and not on your device – but I’ll take it.