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UPDATE: I finally got hold of this problem -> I had in fact 5 desktops activated! Not sure how this came up (and this is something I need to figure out). Basically by looking at the "Activities" I saw 5 different desktops which were live (one being the default). For some reason, the interaction on the desktop area was a bit random with the laptop screen being cloned onto the 24 inches. This is maybe something that the Plasma developer should consider -> make it clear when you have several desktops active (by e.g., showing a list in the drop down menu?). Anyway I killed the 4 other desktops, and I hope that now I only have one (in classical mode) active.
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[Kubuntu 16.04, upgraded from 14.04]
[Plasma 5.5.5 - Kernel 4.4-0-34-generic - 64 bit]

I reported an odd issue recently. In fact I now see that this is a major problem with my plasma 5/kubuntu 16.04 desktop behaviour, causing me major headaches.
I basically see that there are 2 desktops sharing the same desktop space (overlapping).

Initially I had configure my desktop in Folder view. But then I sometimes had all my icons disappearing and realised I was in desktop mode. It's only then that I found out that there were 2 desktops sharing the screen space. It switches from one to the other in very unexpected ways, either by clicking in some areas of the screen, when adding widgets or just after rebooting.

I am positive that I have two desktops overlapping as I see the change in the background image at the bottom of my screen and the mode is also different (one is in desktop mode, the other in folder view mode). Depending on whether I click at the bottom of the desktop area or at the top, I have different set up, different icons etc.

Can somebody help to get rid of this issue?

Note: I am using my laptop and aligned my 24 inches screen with the laptop display. Maybe this is the cause of the trouble (worked like a charm in kubuntu 14.04 before upgrading)? My laptop has a resolution of 1920x1080 while the HDMI1 display is at 1920x1200. So maybe it tries to fill in the lower 120 lines with a second desktop? Not clear.

Thanks for any help you may provide. This is really a disturbing issue and I am thinking now to go back to kubuntu 14.04 if I cannot find a way to solve this.

thanks