"Duplicate these displays" not supported in Boot Camp running Windows 8.1 or Windows 10
When you use Windows 8.1 or Windows 10 in Boot Camp on a Mac with a connected 4K or 5K display in Multi-Stream Mode, the "Duplicate these displays" feature is unavailable.
If you try to use the "Duplicate these displays" feature, you'll get an error that says "The display settings could not be saved. Please try a different combination of display settings."
Learn more about which Mac computers you can use with 4K displays.
Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Contact the vendor for additional information.