Already bought app
I bought an app on my phone, then I wanted it on my laptop, but when I signed in with my same Apple ID it said I had to purchase it AGAIN? How come?
iPhone 6, iOS 11.2.6, MacBookAir 13
Where are you trying to get it from on your laptop ? If the Mac App Store you will have to pay for it if you want it, your phone and your Mac are different operating systems and therefor separate purchases. If the iTunes App Store it should show in the Purchased section for redownloading i.e. Account > Purchased menu option