Windows 7 - Can't duplicate display

Hi guys, I have a Lenovo T510 which I have sitting in a docking station. I have connected an external LCD monitor via DVI, directly into the docking station.

The laptops native res is 1920x1080, the external monitor is 1440x900.

I am having trouble duplicating the laptops display, for a start, the option to duplicate doesn't even show up in windows options, I can only extend or make either of the displays the main display.

I have to press win+p to see the duplicate option, but when I select it, my external monitor shuts down (?) and my laptop pops up a warning saying:

"This is not the optimal resolution for the clone displays configuration. The optimal resolution is 800x600."

Any ideas on what else I could try would be great, I figured it would be a lot simpler to clone my laptops display, but this has been driving me nuts for over a week now.