I was previously using version 2.0.3 (prior to overlay implementation).
Since updating to version 2.2.0 (without using the overlay mode), it seems the form submit event is being fired twice on verification.

I've diffed the versions and can see an additional call to requestSubmit has been added to function verify()

if (auto === 'onsubmit') { requestSubmit(elSubmitter); } else if (overlay) { hide(); }

However, onFormSubmit which calls verify() already fires a requestSubmit to the form

verify().then(() => { requestSubmit(elSubmitter); });

I believe this then causes the double submission.