However, there were two lingering issues:
  - The application auto start list contains both "PolicyKit Authentication Agent for the MATE Desktop" and "XFCE Polkit".
  - Synaptic Package Manager shows that xfce-polkit is NOT installed, which is OK. But it shows "xfce-polkit" under the status of "Not installed (residual config)".

Solution
  sudo apt-get purge xfce-polkit

This successfully removed "XFCE Polkit" from the application auto start list.

Since the original problem was fixed, marking this as solved.

* Test notes, using the 6.1.0 netinstall
  - devuan_excalibur_6.1.0_amd64_netinstall.iso
  * Software selection
    - Devuan desktop environment
    - Xfce
    - SSH server
    - standard system utilities

  + On first login, no errors are displayed.
  ? Strangely, the application auto start list contains both "PolicyKit Authentication Agent for the MATE Desktop" and "XFCE Polkit".
  * Synaptic Package Manager
    + Shows that xfce-polkit is NOT installed. This is the expected result.
    ? However, Synaptic does show "Not installed (residual config)" for xfce-polkit.

  + sudo apt-get purge xfce-polkit
    + Success: "XFCE Polkit" no longer appears in the application auto start list.

I unchecked  the MATE polkit agent, but it rechecked automatically after some reboots.

What can I do?

Remove the damn thing then it has no chance of starting again.

sudo apt remove --purge mate-polkit

Used in a Terminal to do the actual removal.

I unchecked  the MATE polkit agent, but it rechecked automatically after some reboots. 

What can I do?

Thanks in advance.

After a fresh Excalibur + Xfce install, when you log into Xfce, you see an error message box:

  Title: "XFCE PolicyKit Agent"
  Message: "GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: An authentication agent already exists for the given subject"

* Cause

The application auto start list contains both "PolicyKit Authentication Agent for the MATE Desktop" and "XFCE Polkit".

  Applications -> Settings -> Session and Startup -> Application Autostart tab

During Excalibur RC1, I ran several Xfce installations and didn't see any PolicyKit errors on startup. I assume some changes were picked up from Debian since RC1 that caused multiple polkit agents to be installed.

* Workaround

Uncheck one of them. Since this is the Xfce environment, I think unchecking the MATE polkit agent would be better, but otherwise, I don't have any other suggestion for choosing which one to uncheck.

Make sure that you check at least one of them. The polkit agent provides a GUI dialog box for entering your password for running an administrative app. Example: Synaptic Package Manager from the menu, or "pkexec" from the cmd line. If you uncheck both, the default is to request your password from the console output. So if you're starting Synaptic from the menu, nothing will appear to have happened.

* More info

- This problem is already reported in bugs.debian.org.

  #1115933: Error on login: XFCE PolicyKit Agent ... authentication agent already exists