Hello i have a problem with my desktop on dual monitors. As can be seen in the image below. The active monitor extends onto the second monitor. Any idea what could be wrong and how to fix it?

Hello, While I'm a bit new to KDE I have had your issue on more than one occasion which drove me nuts until I figured out how to solve it. And since I didn't see a response as yet for you I thought I'd at least pass along what I found worked on my system. I'm running Manjaro and it has a "Setting" GUI which has a option for Display and Monitor. Under that option I found what I've seen similarly on other distro where you can identify which monitor is which and more to the point graphically arrange to monitors. When I had your problem I finally discovered that even though the graphic of the monitors "side by side" with what appeared to be their edges smoothly against one another..... they actually were not. All I had to do was to drag one of them off to it's appropriate side and slowly move it back close to the other monitor. and when it got close but not quite adjacent I let go of it and they kinda snapped in place adjacent to each other. Hopefully your distro has a similar config tool you might try.
I hope that helps and if not then hopefully someone who knows how to do it with CLI will come along and we'll both learn something.