Cant use dual monitor

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http://askubuntu.com – i have until recently been able to use the dual-monitor function but after restarting the computer i cant activate it again. *I must use Mirror displays or else i get the following error message: required virtual size does not fit available size: requested=(2560, 1024), minimum=(320, 200), maximum=(1280, 1280) How do i change the maximum(fix it) and how can i prevent this from happening again? (HowTos)