Currently, there is no filter or trigger call "Leads were merged."
When leads are merged in SFDC, the leads are automatically merged in Marketo. It would be helpful to see a list of leads that were merged so that we could make sure that duplicate lead that was merged was not placed in any campaigns meant for new leads only. This only happens when a lead fills out a form with a different email address from a non-cookied browser.
It would be even better if we could create a trigger campaign that removes leads, once merged, from campaigns they should not be a part of.