Split screen problem

I have an Inspiron 1564 which had a dead screen. It was showing a black screen which sometimes lit with no display. I tried the laptop with an external monitor and it is showing the correct display (one single dell logo). I purchased a replacement screen (not from dell) but it is giving me two separate displays on the splash screen and in the bios one above the other.

I am told that the screen is compatible (not that I believe everything I am told). Could this be that the screen is wrong or that the two outputs for the display are on separate channels and somewhere there is another fault. If it was the graphics chip then both onboard and external would show double.

Any Ideas?