You shouldn't be able to purchase it again if you already own it. I would log into your Microsoft account and go to your purchase history. Find the purchase and make sure it shows as Completed. If the purchase doesn't show up or has a status other than Completed then you will need contact Microsoft support to identify where the purchase went or didn't complete. If the purchase shows in your history and shows as Completed, was this purchase made on the same account you play the game or do you have a purchasing account that gameshares content to a playing account?