It sounds like the invoice number field has an auto-complete feature, i assume you are utilizing it on a drop-down field populated by that lookup presumably with existing invoice numbers?

I don't think i quite understand the question the way its currently phrased, unless you have another table with a different set of invoice numbers you want to run that check against. In which case the way i'd approach this is by populating a hidden drop-down field with the invoice numbers that already exist, then write a callback function that runs once a selection has been made which will iterate over that hidden list <option> values and check for match. You can then display your error message as a custom bootstrap alert.