If I paid the $1.99 once, will I have to paid for it again if I download the app for a different device?

I actually paid for the in app purchase of $1.99 for my iPhone 4s, but I would also love to have it in my old phone (which is an iPhone 3GS which has been updated) because I use my old phone as an iPod instead. I wanted to know, would I have to pay for it again? I used the same Apple id for both, but when I tried saving it into my photo album on my 3GS , I was asked did I want to purchase it again.