If you set the validation of the field to unique, any records that violate the validation rule are not created, i.e. imported.
I am looking for a function that will allow me to test, in my importing loop
If, OTOH, you don't import directly, but via an intermediate ('scratch') table, simply define a relationship between that table and the Contacts table via the website_url_clean field, and test on e.g.
not IsEmpty ( Contacts_importcheck::id )
which is True if there is already a record* with the same contents in the website_url_clean field.
*One or more.