You don't have to buy it again if you already have the iOS version and the tablet is an iOS tablet (or vice versa). However, if you bought the iOS version and you have an Android tablet, you'll need to buy it again.
Once you pay for the program there is no need to pay again as long as you log on to iTunes using the same login that you purchased it for. I did the same thing after I bought my Apple Air 2 iPad. Once you hit the BUY button, the next screen should tell you that you have already paid and therefore will be downloaded at no cost. Those were my steps when I did it. Good luck and you'll love flying with the Air 2.