There's one form with information about each title (title, subtitle, series name, series number etc) and I need to assign multiple ISBNs (identification number) to each title depending on format (hardback, paperback, ebook, audio etc). 

I've got hundreds of ISBNs in a CSV to import, so I put these into one form. The title information is in another. I tried assigning them with a lookup, but have run into issues in how to make sure the ISBNs don't get used more than once. I tried using a formula field on the ISBN form that was checking whether the title lookup field was empty or not, and then filtering by that field. But it doesn't work because the formula doesn't update fast enough so when I go to assign another ISBN the previous one is still available through the lookup. This is roughly how it was done in Filemaker, but they have dynamically updating calculation (formula) fields.

Anyone have any idea how to do this?