The best thing to do is to contact support to request a refund for any repeated charges.

Many games allow you to donate multiple times and you are probably clicking the buy button again and paying again.

When you have already paid for a game, when you open the game page, at the top you should have an option that says "download".

If not, you can click the "buy" button, but you must click on the bottom where it says "I already pay for this", if you click on paypal, you will have to pay for the game again.

The third option, and the one I consider the simplest, is to click on the arrow to the right of your avatar (top right corner) and there in "My library", there you will see a list of all the games you have purchased and a download link.

EDIT:

If the game doesn't show up as purchased in your library, it may have been an error (you should see an error message during the billing process). It's possible that the charge was flagged for you, but the transaction didn't go through, in which case you wouldn't have the game, and you do see the charge, but it's usually automatically cancelled after a few hours.

Still, if you have any questions about the charge, it's best to contact support.