Windows 10 Pro 20H2 build 19042.844 On Dell Laptop with Dell Docking Station.
Two Monitors using HDMI input, one from laptop and 2nd one from Docking Station.
When I select DUPLICATE on 2 and 3 it only Duplicates on 2 and on 3 it Extends.
Not seen this before ... Only thing I can think of is what Graphics card are you running from?
Although the signal for the 3rd monitor is coming Via the Docking Station, the video is actually coming from the Laptop - It may be that the Graphics system is only capable of driving two screenfulls of data at the requested Resolution & Aspect Ratio so does this and sends a duplicate of the data going to Monitor2 and a extended screen Monitor3, [this lowers the stress on the system to allow it to drive both additional monitors]
You could try swapping the Monitors around and see if the same issue happens.
Another option is to lower the resolution on the monitors [Especially Monitor2 as this appears to be a high res device compared to the other two
It could of course be as simple as the difference in the aspect ratio on the two monitors, Monitor 1 and 2 look to be 16:9 but Monitor3 looks like a 4/3 - It's difficult to display a 16:9 image on a 4:3 screen unless the software/hardware is designed to do this [Letterbox format in the Monitor3 Device controls]
One simple way to achieve one main and two duplicates is a HTMI "Splitter" cable [The old Y cable out of the Laptop or Docking Station - This may or may not give a degraded image depending on the signal strength
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