The default sql addressbook in Roundcube has functionality that makes sure new group (or renamed group) name is always unique instead of throwing an error. E.g. If you have group "My Group" and you try to create another with the same name, Roundcube will just create group with name "My Group 2".

So now, which is better, the above solution or just error? If we decide the error is better, we should probably also change the default addressbook behaviour.

But it is not shown in the addressbook as "My Group 2". Only "My Group" is shown but with the Content of "My Group 2".

I was talking about default behavior of sql addressbook (which you do not use), just to show possible (counter) solution.

There's no technical provision that prevents us from allowing users to create multiple groups by the same name, provided that the data-to-presentation application logic uses keys that uniquely tie an entry to a (specific instance of a) group. Both SQL (primary index auto-increment) and Kolab (xml/uuid) provide these artifacts.

I don't see a particular use-case for multiple groups with the same name other than in a shared folder scenario (albeit the path to the folders that contain the groups with the same name would be different), but there's certainly no technical limitation to allow the users to shoot themselves in the foot should they choose to.