Program Builder - Allow members to enter the program multiple times

I would like to create a doppel opt-in program with the program builder.

I want that all contacts with a specific criteria recieve an email with a confirmation link to the newsletter subscribe.

It's important that each contact goes only once through the program.

How can I make sure that this will happens?

I'm really confused about the point "Allow members to enter the program multiple times".

What exactly does that means?

I understood that this field must be selected that my contacts will enter the program. Is that correct?

And does it means as well that my contacts will go through the program again and again?

Thank you for an answer.