When attempting to switch displays to Duplicate mode on a Windows 8.1 system using the Projection (Win+P) option on a system that has a 4K monitor connected through DisplayPort (DP), the display may not switch to Duplicate mode.
Windows 8.1 may not yet fully support certain 4K (3840x2160) monitors with DP 1.2 enabled. When set to DP 1.2 mode, this type of 4K monitor presents itself to the graphics adapter as two separate 1920 x 2160 displays at 60 Hz rather than a single display.
If your system has an Intel graphics adapter, use the Intel HD Graphics app to configure for Duplicate (Clone) mode. If you have a different graphics adapter, contact the manufacturer for support.