Redirect to url if duplicate value submitted via special token

I love the flexibility of webforms but have run into a snag that I am hoping someone can guide me in getting resolved:

I am making use of special tokens to get certain unique values prepopulated into a form from another site.

Naturally and correctly if a unique value is entered a second time into the webform (i.e. same person trying fill in the form for a second time) an error message is displayed to the person trying to fill in the form.

Instead of the error message being displayed can I get the form to automatically redirect to the site login page?

Any help will be highly appreciated.