It shouldn't be possible to purchase the same app twice in the Play Store under one Google account, so most likely one of the transactions was cancelled (either by you manually or by Google Play Store) and the money will be returned to your credit card soon.
If you see two charges on your credit card it might also be because it's a pending authorization as described by the screenshot below, taken from Google's Web Store FAQ.
Either way, we don't have much control over the payment operations, from our end we only receive the money once.
If you find that after a week or two you still haven't received your money back, it would be best for you to contact Google directly by going to Google Wallet > help > live chat. Though so far in our experience, it usually doesn't go so far as to requiring this step.