Comment 3 for bug 1925998

The behavior of juju 2.9.22 is now different. You cannot explicitly set a variable to the same value as default for the charm:

to demonstrate:

$ juju deploy ubuntu
$ juju config ubuntu hostname=""
WARNING the configuration setting "hostname" already has the value ""
$ juju config ubuntu
*snip*
settings:
  hostname:
    default: ""
    description: Override hostname of machine, when empty uses default machine hostname
    source: default
    type: string
    value: ""

Note that the source remains default, and the user cannot explicitly set the value any longer. This protects against bundles that overly zealously configure charm settings which are default, but doesn't protect against charm authors potentially changing charm defaults in the future when users would prefer to nail their configs to the current defaults as they upgrade their charm.