2nd Monitor not detected (Dell Xps 15 / HDMI)

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http://forums.fedoraforum.org – I'm trying to connect my notebook to an external monitor. However, the monitor is not connected. My notebook is a Dell Xps 15 and the screen is connected via HDMI (DVI on the screen and HDMI on the notebook). I also tried disper -l and it just lists the notebook's screen. Any ideas? (HowTos)