Allow it to restrict to only one opportunity in one pipeline, while allowing duplicate in others.

For instance, I don't want multiple SALES opportunities created but I do want multiple SUPPORT "tickets" created.

I would think that if you turn off DUPLICATE OPPTY in company settings....... but in the workflow where there is a checkbox to TURN ON duplicate opportunities that would override the company setting.

I would only be creating those duplicate oppty via workflow when someone submits a form.