How to mirror displays with my laptop and an external HD Monitor

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http://askubuntu.com – I have been trying to connect my laptop to an external monitor (different resolutions). My idea was to setup the external monitor as a clone of the laptop screen. In Ubuntu Quantal's display settings I can detect both displays but I can only setup as an expanded dual monitor. The mirror option is visible but disabled. Is there a way to "enable" mirror mode? (HowTos)