Clearly you dont have a Server with more than 100 Domains and Customers where most of them have the need to name at least one Mailinglist with the same name.
For a Server-Software with support for Multiple Resellers, Customers etc. it seems like a shame. ;-)
mailinglists are handled by mailman and that uses only the part before the @ as the mailinglist name. This is not a plesk problem, its just the way mailman works.
The only solution to fix this is to install a mailinglist program on the site itself. We always recommend phplist and customers can install it via our installatron script installer.