Do not allow duplicates in one list only

[jgoth]

Is there a way to keep duplicate users from signing up for the same list?

[DW]

It is the default behaviour of ListMail to prevent duplicates on a per-list basis.  When a user tries to subscribe twice they should be shown the "Duplicate email" error message from the error message set applied to your list.  Are you seeing duplicates in your database?

Regards