Dell C5517H - Setting Display Styles for Multiple Monitors

64  Setting Up Your Monitor
Setting Display Styles For Multiple Monitor in Windows 7 or Windows 8/
Windows 8.1
After the external monitor(s) have been set up, users can select the desired
display style from Multiple displays menu: Duplicate, Extend, Show
Desktop On.
• Duplicate these displays: Duplicate the same screen on two monitors
with the same resolution, chosen based on the monitor with a lower
resolution.
For Windows 8/Windows 8.1