Created attachment 3224 Screenshot of the two overlapping wallpapers I'm using Xfce 4.8 pre1 / xfdesktop 4.7.2 in a cloned dual screen setup. Using two different resolutions results in two overlapping wallpapers on the bigger screen. See attachment.
On a multi-monitor setup, xfdesktop will now avoid drawing over the wallpapers that came before it which should help some with these issues, depending on the order you have the monitors setup: http://git.xfce.org/xfce/xfdesktop/commit/?id=963d829f123b7cd4da36c25fa03cf5754897e195 This fix will be available in the next stable release, you can test it in the 4.11 dev release (or in master). Let us know if this is fixes the issue for you.
Fix is already in master, Closing bug report.