Monthly billing is based on the sum total of subscribers you have in each of your lists in the account. As each list in your account is independent of other lists and do not share any information, the monthly plan list size does count duplicates each time they appear.
Monthly Accounts:(Under 50K)
If your list totals goes over the limit for your selected monthly plan, you’ll be automatically upgraded to the next plan.
Similarly, if your list totals have decreased to under your monthly plan’s minimum, you’ll be downgraded to a lower billing plan.
High Volume Monthly Accounts:(Over 50K)
If you go over the number of emails you can send on your plan, you will automatically be bumped up to the pricing plan that meets the number of emails you need to send per month.