Allow multiple entries of every property for contacts

Is your feature request related to a problem? Please describe.
I am unable to add multiple timezones for contacts when I want to record that they utilise multiple timezones. For instance, UTC and another, if a developer, or London and Spain, if they possess multiple residences they are routinely at. It prevents me attempting to call them during undesirable times. Additionally, multiple Detailed Names are useful because a person may possess different registered names at different countires. This is legally useful. Mutliple birthdays, too, are useful when I am not yet certain which birthday is correct, but want to be reminded of both, especially to ascertain which is correct.

Describe the solution you'd like.
Allow every property, not merely some, to be possible to duplicate.

Describe alternatives you've considered.
Utilisation of the notes, but this is barely a solution, if one, because it negates the benefits of predefined properties: no reminders for birthdays, or one-click communication via e-mail addresses, and no synchronisation of values.

Additional context.
When displaying names to synchronised software and within the interface of Nextcloud, names may be expected to be without duplication. I propose that there be a check-box to allow one of the names to be primary, because it easily and ideally solves that problem.

The issue does not name any files, tests, or entry points. Start by locating the server code responsible for contact properties and synchronization, then determine how duplicate properties and a primary name would be represented. Done means supported contact properties can be duplicated and the primary-name behavior is defined and tested.