[Koha] Autobar code question

Hi, I am trying to move to Koha and have imported our catalog at this point. In Koha administration if autobarcode is set to "generate in the form of 1,2,3" does Koha simply start and 1 and count up or will it make sure that the barcode it is generating is not already in use (from my imported records) and keep incrementing till it finds an unused code? Thanks, Shawn