While I agree with pretty much everything said here, let's bring this back from a general discussion on candidate keys and the like to answering the original question from Cindy shall we?

Cindy,
You're dealing with patients right?
If you're in a hospital or something even remotely resembling a hospital, I'm guessing that there's already a way that the patients get identified. If there is, then use that.
That way, if you've got duplicates, at least they'd be consistent duplicates, and everyone would have a problem, not just you.

If you don't have a pre-established identifier, then I'd suggest using an Autonumber.
You'll probably find that you get duplicates in any event, but regular checking should help to overcome it.
If you find duplicates based on varying combinations of a few fields, such as:
Surname+FirstName+SSN
Surname+SSN
SSN
DOB+Surname+FirstName
DOB+Surname
DOB+FirstName
DOB
Surname+DOB+************
Surname+SSN +/- 10
FirstName+SSN +/- 10


then you should be ok



I am a loud man with a very large hat. This means I am in charge