We do have double opit-in on all pages enabled.
What happens: A contact gets with double opt-in receives an marketing e-mail. When the contact (re-)submitts a form by entering a different e-mai address, the new address is correctly added to the existing contact as new primary address.
However, the opt-in request e-mail ist not sent to the new e-mail address. All future marketing e-mail sends fail because of the missing double opt-in.
In my opinion, the built in Double opt-in process should also work in this use case, where a contact enters a second/new email address on a form. The opt-in e-mail should be sent to the new address, so that the contact is asked to confirm it.
