I've confirmed with support that currently, when a duplicate email is added to a list, even if it's already in subscribed status, a confirmation email still gets sent.
For our use case and many other businesses that take emails as part of a process, it would be helpful to have the option to not send a confirmation email if the submitted email is already in a list, especially if they are already subscribed. Hoping this is not too difficult and thank you for your consideration.