[Monitor] Weird dual screen bug

Today I was testing my main monitor (a DoubleSight DS-305W, 30", 2560x1600) on another computer. With my computer on, I disconnected the DVI cable, plugged in a DVI-HDMI adaptor, plugged in the other computer.

It wasn't recognized by the other computer. When I was to put it back on the original computer (without the adapter), the screen was duplicated! I tried to reset the machine, with no success. I know it's not software because the bios post is also duplicated, I know its not the DVI cable, because I tried another one that I know works in that same monitor, with the same result.

Any ideas on how to solve this problem?

My machine is a i7-3970k, with a Biostar motherboard and a GTX 560ti video card.

Thanks!

A photo of the problem: