Disallow multiple "reservations" off of my "inventory" list

Hi all, first post!

I have an app with an inventory of items. Users are allowed to “reserve” some quantity of each item. From my inventory tab, I open a form that writes a row to a “reservations” tab.

What I want to prevent is users being able to reserve the same item again… basically each row in the reservations tab should have a unique item-user column pair.

Right now I am fixing multiple reservations of that kind via a timed spreadsheet script. Not ideal as it still allows users to make these multiple reservations, affecting the available quantity for other users until the script triggers.

How can I selectively allow a user to make a reservation based on them NOT having a corresponding row in the reservations tab?

I feel like “user specific column” on the inventory tab would really solve this, but I know I’m not allowed do much math with those data in that column.

I may be butchering this description, but don’t want to type a novel that probably won’t help things. Happy to follow up with more details. I feel like there is a reasonable solution somewhere. I’m also happy to read blogs or watch videos if anyone thinks a particular one will help.