if all entries need an exit I’d structure it like this

inventory item - name etc
inventory_entry - inventory, date_in, date_out

then a user would submit a form which create inventory_entry and adds the date_in date.

user could select the entry in a repeating group and then add the date_out (or you could use buttons to simplify)

this gives you the ability to do a simple search - search for inventory_entry where date_out is empty

you could also have multiple inventory_entries for the same item that have not been checked out yet.

given your explanation above this is what I’d do, but I don’t quite get the purpose of it so it might not be the correct data structure