Macdguy has already stated before that you will be able to buy milestone items you already unlocked so you can own multiple of the same items if you want them.
As much as you can afford really.
The whole point of being able to purchase milestone items you’ve unlocked is so you can own as many as you need / want.
This came up again in the Discord so I just wanted to bump this post to add that the Upgrades Store is said to work very similarly to how OP described buying milestones. Because of that, I think the game could already support locking purchases behind an EXP requirement, so I’d really like to see a Milestone Store happen at some point. I just want my 16 Derby Horses
I don’t recall if there’s ever been confirmation on being able to buy milestones from the Upgrades Store or not, which is why I bumped this thread. What I do remember is that the Upgrades Store will also house a shop for each gameworld where you can buy props like the Ball Race bumpers for example.