Duplicate monitors in dialog for desktop icon layout

Issue Description

As mentioned in #420 (closed), the dialog for configuring the desktop icon layout for a monitor shows each detected monitor twice. If they are not the same, at least the monitor names or an indicator should give more information on how/where they differ.

Steps to Reproduce Issue

1. Plug in a second monitor for which a desktop icon layout is not yet configured

Expected Behavior

The monitor selection list contains no duplicates.

Additional Information

Environment & Versions

• OS distribution and version: Arch Linux
• Overall Xfce version: 4.20
• xfdesktop version (git hash if not a release): 4.20.1
• Windowing system (X11/Wayland): X11
• Compositor and version (if Wayland): -
• GTK Theme: Arc-Darker
• Icon Theme: Faenza
• UI Scale Factor: 1.0