Windows 10 monitor issues with 5k iMac

[otomo]

Just got 2017 5k iMac18,3 and there are a few issues in Windows 10 Pro Version 1709:

1) In Device Manager I see *two* Generic PnP Monitors. What is the second monitor?

2) When I send HDMI from the iMac to my a/v receiver (now I see *three* Generic PnP monitors) the resolution setting of the main 5k display becomes limited to 3840 x 2160. When I disconnect the a/v receiver, the 5120 x 2880 resolution is again available. Can't I use the a/v receiver for 7.1 sound and keep my 5k resolution at 5k?

 

[otomo]

Okay, I accidentally "fixed" it.

1) I went into Radeon Settings and then into Radeon Additional Settings and accidentally grouped the main monitor and the phantom second monitor into an "Eyefinity" display.

So is the iMac's 5210 x 2880 display actually two portrait 2560 x 2880 displays grouped together side by side into a landscape? I don't get it. From what I've gathered, Eyefinity is for grouping physically separate displays, not two halves of the same display.

2) The accidental Eyefinity arrangement fixed #2.