https://itch.io/docs/creators/pricing
Seems the faq is ill worded. They use the term individually priced files, but they are not. It is just a non increaseable payment tier to implement paywalls. And that is quite a problem in itself. As you counterintuitively cannot stack dlc on top of your game to encourage people to come back and increase their payment.
That you can "purchase more than once" on the same account is a problem. It bascially should not be possible. Like, legally.
"Before we talk about pricing, we should mention briefly how ownership works: Whenever someone pays for your project, they become an owner of it. When granted ownership, they are given a unique download key that grants them access to the files uploaded to the page."
Selling you stuff you already own sounds rather illegal.
This is what steam says to you, if you try: "Your account already owns some of the above items, so you can’t purchase them again." (Gifting it to someone else is possible).
Since you can overpay, there should be a method to overpay later. That you can pay again feels like a hack and not working as intended. And if it is intended, someone should think again.
The drm (and any "piracy" prevention) in online games is implicit. You need to log in. No login, no play. And if logged in, you apply the "rights management", as in, only users that have paid can access the content.
My guess is, it would be some kind of tos violation if you require people to purchase the same project several times to be able to play. Itch is optimised for downloadable games, and it shows.
They should at least implement the increase of payment tier. That feature is needed in the existing framework. Some recurring payment options sure would be nice for some projects, but might be not worth the efforts and risks. There is a slight difference in user-initiated payments and collected payments, would be my guess.