unable to mirror display

I connect an external monitor to my Dell inspiron 1720 laptop.
I open the windows mobility center and choose to add an external monitor
it detects the monitor
I choose to "mirror" the display and all it will ever do is reset the
resolution on my laptop display and it does not ever go to the external
monitor.
I did a google on this and found others with different machines who are
having the same trouble. I was not able to find any resolutions however.
Can anyone offer suggestions or a fix?


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