Lot of questions about this today 
Your IAP is tied to your iTunes account, so if you delete and reinstall the game or move to/share on another device, you will not have to pay twice as long as you’re using the iTunes account which initially bought the IAP.
However, to redownload the IAP you’ll have to go through the purchasing process again, which only checks at the very last minute whether you already own it. Select ‘Buy’ then agree to pay on the pop-up and enter your iTunes password if necessary. Only then will you get a pop-up saying you already own the content and it’ll be downloaded for free.
Hope that helps!