If you are importing a list, they will import as single opt-in and do not have to opt in again. We evaluate each upload using both an algothim and manual checks to approve them for mailing. We trust your acknowledgement that the contacts are valid, and are expecting to receive email from you. We can tell from the bounce and complaint rates when someone is using a list that isn't expecting email, so we don't make everyone have their entire list opt in again.
If you have a list that was double opt-in and you want them double opt-in for ONTRAPORT as well, you will have to ask them to double opt-in again. In order to qualify for the double opt-in service, where the email is sent from only double opt in servers, the contact must double opt in while receiving email from ONTRAPORT.
We make it very simple to send a one off double opt-in to your contacts once imported! You can send the double opt in message by checking one or more contacts (or even all of them!) and selecting the Double Opt-In item from the action menu: