Multiple duplicates being created from web-to-lead

Tought to know the what to look for, here are some ideas:

• Start by trying to duplicate the issue by submitting the web form yourself a few times to see what happens in
  Salesforce.com. If you can't get it to create dupes, then it could be that the user is double-clicking the [Submit] button on the form, or possibly clicking it multiple times. That could be resolved with some javascript on the button to disable the button upon clicking.
• Something else to check might be the returl set by the form when submitting to make sure there isn't a circular reference of some kind going on that resubmitting the same form.

I'll add that standard Web-to-Lead functionality will always inserts a new lead, regardless of whether the person exists already as a Lead or a Contact. So, if for some reason the form is somehow being submitted multiple times, it will create multiple leads.