SUMMARY
Placing a window between displays (horizontally or vertically) will cause the window to appear on each display it appears between. For example, if it is placed between two displays horizontally, it will appear in the overview effect on both. If it is placed between four displays, it will appear four times. This occurs no matter the display orientation.

This is purely a visual glitch, clicking on any of the instances of the application will focus it correctly.


STEPS TO REPRODUCE
1. Place a window between two or more displays 
2. Activate the Overview effect

OBSERVED RESULT
The window appears on the Overview effect for all displays that it overlaps.


EXPECTED RESULT
Overview effect only appears on one display, perhaps the display it was most recently focused on, or the display that contains most of the window? But I'm not sure how that would work in the case of a setup with vertical and horizontal displays. I'm not sure what the most intuitive solution would be here, apologies.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.16.8-arch1-1 (64bit)
KDE Plasma Version: 5.24.0
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Only tested on Wayland, unsure if this affects X11.