When you create domain alias, Plesk allows you to create alias with www prefix. For example, creating alias with name www.alias.tld in Plesk, will create DNS record as below:
www.alias.tld. A 184.108.40.206 www.www.alias.tld. CNAME www.alias.tld.
How to avoid creation of duplicated prefix like www.www.alias.tld? Plesk should show warning telling that www should not be used during alias creation as Plesk create it automatically.
Enhancement request PPPM-1946 is created and will be implemented in the future Plesk version. Plesk will show warning about duplicated www records if client tries to create alias with www prefix.