And do you see both charges when viewing your purchase history?
If so, check in the game as usually there is an account section that will show your previous purchases. If there is a discrepancy there with what it shows in your purchase history, then screen shot those pages. First contact the developer and preset them with the proof. They can't issue a refund, but they can give you a free promo code that you can redeem in the App Store or update your account on their servers to reflect the purchase.
If no luck with the developer present the information you have to Apple along with what the developer told you and request a refund here. In most cases all transactions in the App Store are final, but on occasion Apple may issue a compassionate refund.
Request a refund for apps or content that you bought from Apple - Apple Support