If you wanted to do it clientside, you could write some kind of javascript, like onClick=disable(this), or whatever the correct js is. There should be a way of switching off the button.

You could put a (random) hidden varible into the form and keep track of which 'numbers' have just submitted data in a local file. Clear the number after you've finished processing their form and sent the response, or at midnight, whatever takes your fancy. Or if you're pushing data into a database you could do a query to see if you're submitting duplicate information.

Or, at the risk of getting shotdown by the perl fanatics here (i.e. almost everybody) use the persistent (or session? what are they called?) variable features of php4.

I know there are a few other ways but I haven't had to tackle this problem directly before..

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Jeremy