Create a key generation algorithm so that each subject not yet in the system gets an unique entry. Try not to use any changeable information (name, age, etc.) for your primary key.
An additional benefit is that you can archive old data and retain it, which may be helpful if and when you fingerprint a subject wearing a fingerprint overlay of someone else, for example.
But you have no doubt solved this problem in the four years since asking so please Improve This Answer with the solution you selected in the end.