Segments are not necessary to prevent Acymailing from sending the same email twice to the same user. When an email is sent to multiple lists, the subscriber will only receive it once, even if they are subscribed to multiple lists. If you need to resend the same email, Acymailing will prompt you to send it to all users or only to users who haven't received the email yet.
However, if you want to target a specific group of users who have received a certain email before, you can use a segment. An example would be sending a reminder to users who have previously received your call for donations.