authentication_agent_initiate_challenge potentially duplicates user_identities

[hsitter]

Describe the bug
The same users (as per its hash and equal function) may appear multiple times in the identities list sent to authenticators. This appears to be the result of authentication_agent_initiate_challenge operating on a GList and not checking for duplicates nor running a uniqueness function on the result.

To Reproduce
Kinda terrible to reproduce but I've seen this across multiple distros

1. on arch linux with kde plasma
2. systemsettings kcm_users
3. create new admin user
4. pkexec ls

Expected behavior
Every user appears once

Screenshots

Desktop (please complete the following information):

• OS (including version): KDE Linux 202501060256
• Desktop Environment [Gnome, KDE, ...]: KDE Plasma Version: 6.2.80
• Version of polkit: git main

[sbijendra]

Recently, we also upgraded our ubuntu version to 24.04 and looks like we are also seeing the same problem now where we are seeing multiple admin users while doing a restart on some service. We are seeing admin user 2 times as admin user is part of both admin and sudo group in our use case.
`Multiple identities can be used for authentication:

1. admin
2. xyz
3. admin
   Choose identity to authenticate as (1-3)`