How to avoid the subscribtion of those visitors who are already on my list?

In order to avoid having subscribers' data twice on the list, please do the following: within the campaign setting find the "Visitors who have not visited a page during the recent period of time" option, click on "Add New" button, select equals and add the URL of that thank you page, to which your visitors are taken after the subscription.

In this case, those returning visitors who have already subscribed previously will not see the popup and will not have a chance to subscribe once again.