[I've just emailed them this issue - I'll post here if I have any answer]
I have 4 lists.
My form adds the subscriber to a default list - then he/she receives a confirmation email [so far so good].
If the subscriber, on the the subscription page, subscribes to any other list, he/she will receive a confirmation email every first time they do so - that's up to 3 additional confirmation emails they receive.
I'll have to roll back to the previous version until this is resolved.