Many thanks Fou-Lu..
This is probably why I couldn't work out how to do it.
I will have a look at your suggestion and see what I can make of it.
Failing that I will have to figure out another way of dealing with duplicates, or even accept duplicates.
My reason for not wanting to accept duplicates is so that if I send out a mail-shot, I don't want to send it to the same email address a number of times.
At the same time, I don't want to restrict the number of times that a user can submit their email address in order to get what is on offer.
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