When an operating-system contains multiple users or groups with the same name, instantiating it with `guix system` does not cause a validation failure, nor are the duplicate entries filtered from the resulting /etc files.

This duplication can happen in a few different ways:

- both entries are manually included in the "users" or "groups" fields of the operating-system
- a manually-specified entry collides with an entry defined by a service (via an account-service-type extension)
- multiple services define entries that collide with each other

Steps to reproduce: call "guix system container" with the attached operating-system definition.