If you have the following 2 settings:
- "Membership Behavior" set to "Quantity Creates Different Memberships"
- "Limit to one membership" set to "Yes"
then you can have a customer with 10 email boxes, and the scenario described earlier will keep their quantity at 10, and not double it to 20.
We also updated it so that when you pay an old membership order, it will change the "Next Charge" date according to the old "Next Charge" date (the date the old invoice was created) plus the number or days it is meant to recur. I believe this will address the issues you mentioned earlier about the charge date.
Please let me know if there are still issues, or if you have any more questions!