Thanks, David (and Sergey)-

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>Since random numbers on digital computers are not really random at all, you should generate the sequence all at once and then just use numbers from that pool and delete them as you use them.

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I hadn't considered the possibility of generating the numbers first, and that absolutely makes sense. I'll do it this way. I appreciate your expansive description of your task. My situation is somewhat similar, though not as sensitive. However, I do want to take a minimum precaution against people just guessing a valid serial number. Thanks very much.