Above is a video of what will actually happen. Spoiler - it won't charge you a second time. Apple has acknowledged this issue in a support request, but it is doubtful that it will be fixed, as the issue has persisted for years.
April 21, 2025
Issues on the Mac App Store
After purchasing Patterning 3 on the iPad, the Mac App Store often show a price to download the app on your Mac. This is a bug on the Mac App Store.