However, your earlier purchase might not have been recognized, for one of two reasons. This would result in an extra charge when you try to purchase it again.
The first possible reason is that you purchased the in-app upgrade to the free version in the past, and this time you purchased the Pro version (or vice versa). Each of our topics comes in two versions: an upgradeable free version and a pay-up-front Pro version (with a yellow "Pro" banner on the icon). The upgraded free version and the Pro version are equivalent, but not interchangeable.
The second possible reason is that you may have inadvertently created an additional App Store account in the past, and you were signed in to one account at the time of the first purchase and you were signed in to a different account at the time of the second purchase.
If you inadvertently purchased both versions, please contact support so we can help you with a refund of the extra purchase.