Unique records for mass mailing

This module adds uniqueness rules to the Mass Mailing app so that you cannot have two mailing lists with the same name or two mailing contacts with the same email address.

• On mailing lists (mailing.list), the list name must be unique. If you try to save a second list with an existing name, Odoo shows the message "Cannot have more than one lists with the same name."
• On mailing contacts (mailing.contact), the normalized email address must be unique. If you try to save a contact whose email already exists on another contact, Odoo shows a message telling you to delete the existing record or change its email.

These rules are enforced at the database level, so they also apply when importing contacts or lists in bulk, helping you avoid conflicts when importing contacts into a list whose name is duplicated.

Before the module is installed, it checks the existing data. If there are already mailing contacts sharing the same email or mailing lists sharing the same name, installation is stopped with a clear error listing the duplicates, so you can clean them up first. The module does not add menus, buttons, reports, or scheduled actions; it only strengthens the existing mailing list and contact screens with these uniqueness checks.