Currently display window all supported modes. Now ("wide"-type, 16x9 monitors becomes rather widely spread) it looks as not good idea: geometric distortions make standard (4x3) modes look ugly on wide (16x9) monitors and wide modes on standard monitors. I suggest to expand monitor mode string with monitor-type selector (values: 4x3, 16x9, show all). And an issue: $ xrandr Screen 0: minimum 320 x 200, current 1440 x 900, maximum 8192 x 8192 VGA-0 connected 1440x900+0+0 (normal left inverted right x axis y axis) 410mm x 256mm 1440x900 59.9 + 75.0* 1280x1024 75.0 60.0 1280x960 60.0 1152x864 75.0 1024x768 75.1 70.1 60.0 832x624 74.6 800x600 72.2 75.0 60.3 56.2 640x480 72.8 75.0 66.7 60.0 720x400 70.1 DVI-0 disconnected (normal left inverted right x axis y axis) xfce-base/xfce4-settings-4.10.0 But in XFce Display list shows 1440x900 mode two times.
Marking as WONTFIX for the feature request. There are too many display ratio variations, and displaying all supported is preferred. If you're still having the duplicate issue in xfce4-settings 4.11.3, please create a new bug report.