Recently experience this at work and thought it would be worth posting incase anyone else experiences the same problem.
If you have dual monitors and have choosen "extend my windows desktop onto this monitor" you may find that wallpaper is duplicated on both monitors instead of spanning across them.
Turns out there is a solution to this, not obvious, but real simple. When you select a new wallpaper, it defaults to 'Stretched' mode, however if you change it to 'Tiled' mode the problem is sorted !
Now you just need to resize the image file so that it matches your monitors resolution.