>>They'd better be, otherwise...As Jim Booth and Steve Sawyer pointed out in their "Effective Techniques..." book, not everything that we assume to be guaranteed to be unique are. The example they used is Social Security Numbers. There have been instances of duplicates among living persons.
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>My #2 kid went through much of his life using the wrong SSN. That would easily cause duplicates if the other person with that number happened to bank in the same place or use the same physician.

Data entry errors are a source of frustration for any would-be primary key. Surrogates are the best way to go.