Allow more duplicate fields.

Is your feature request related to a problem? Please describe.

I currently use MementoDB for storing contacts of any marginal complexity. I don't want to do this, because the relevant database software doesn't interact with the local Android contact storage nor provide a CalDAV server. Nextcloud can.

However, I am unable to transfer many contacts, because I am unable to duplicate fields such as the name field. This field, for instance, needs to be present multiple times in order to support previous names for a contact, lest I forget who the contact is due to a name change. This problem is common for me.

Additionally, many contacts have multiple managers, as https://github.com/nextcloud/contacts/issues/3826#issue-2152385220 explains.

Describe the solution you'd like

Allow more fields to be duplicated.

Describe alternatives you've considered

I transfer all of my contacts to a more versatile database and engineer a CalDAV synchronizer. Not gonna occur.

Additional context

Considering that certain fields – e-mail addresses and telephone numbers – already allow this, I see no fundamental reason why this should be even vaguely difficult to implement. If for whatever reason it breaks CalDAV synchronization, just allow the user to set one of them as the "primary" option.

No file or test is named. Start by reviewing how existing duplicate email and telephone fields are handled, then inspect the CardDAV synchronization implications and the referenced issue #3826; done should support repeated name and manager fields without losing synchronization or primary-field behavior.