So I have a regular 21" monitor as my main (mon1), a 19" wide as my secondary (mon2.) Naturally they have different resolutions.

For the new version (3.1.5) with Taskbar on each Monitor I'm seeing the background of mon1 "inside" the toolbar on mon2. If I make a straight line across mon2 to mon1, it so happens that the part of the background I'm getting is exactly where the bottom of the monitor is physically located.

Mon1: 1280x1024
Mon2: 1440x900

Better still, here's a screen shot..

But... in previous versions it wasn't so... noticeable. I'm still using the same wallpapers. I mean the wallpaper duplication doesn't even make it across the bar, it stops where the chrome button is on the taskbar.