If you are of the lucky guys who owns an nVidia based Video card, you can achieve what you wish by using the nVidia xServer Settings. I am assuming that you have the latest nVidia Drivers installed, so, try setting up your nVidia Settings by dropping the next in a terminal:
sudo nvidia-settings
After entering your password the nVidia X Server Settings window will appear, click on the "X Server Display Configuration" in the left pane and you will have the options to choose between: twin view (clone), extended desktop or simply turn off a monitor, after changing something, clic the "Apply" button in order to perform a test and don't be afraid if your monitors flickers. When you are satisfied with the configuration just save your changes by clicking the "Save to X Configuration File" button.
IMPORTANT NOTE: either that you can drop the single nvidia-settings command in the terminal, it won't allow you to save the changes. Do it as suggested and let's see what you get.
EDIT: I was writing my answer when you said that you own the Intel Based Graphics Card. So, I would suggest you to use the System/Preferences/Monitors tool, which should allow you to do what you wish. In all cases, make sure you have connected the external monitor before doing this, sorry if my first answer wasn't helpful for you. I didn't wish to erase it as this can be helpful for somebody else. Thank you.
The specs of your Dell Latitude reads that your system may have one of the following:
If you are of the lucky guys who owns an nVidia based Video card, you can achieve what you wish by using the nVidia xServer Settings. I am assuming that you have the latest nVidia Drivers installed, so, try setting up your nVidia Settings by dropping the next in a terminal:
After entering your password the nVidia X Server Settings window will appear, click on the "X Server Display Configuration" in the left pane and you will have the options to choose between: twin view (clone), extended desktop or simply turn off a monitor, after changing something, clic the "Apply" button in order to perform a test and don't be afraid if your monitors flickers. When you are satisfied with the configuration just save your changes by clicking the "Save to X Configuration File" button.
IMPORTANT NOTE: either that you can drop the single nvidia-settings command in the terminal, it won't allow you to save the changes. Do it as suggested and let's see what you get.
EDIT: I was writing my answer when you said that you own the Intel Based Graphics Card. So, I would suggest you to use the System/Preferences/Monitors tool, which should allow you to do what you wish. In all cases, make sure you have connected the external monitor before doing this, sorry if my first answer wasn't helpful for you. I didn't wish to erase it as this can be helpful for somebody else. Thank you.
System -> Preferences -> Monitors CHeck "Same image on all monitors" Apply Close