Suddenly my mouse is all screwed up: a single click registers as two clicks - one for the press and one more for the release,
or at least looks like that, but most of the time i click once and it treats it as two distinct clicks. i tried to fix it in the “configure desktop” but no go… It’s v11.3., kde4
Did you reboot
Of course. This started a week ago.
Try a new user login
See if it’s the same
It’s global.
I remember this happened before and I upgraded to v11.3 which fixed it.
But upgrading is now ( always ) a fix …what can it be ?
Mouses do fail over time. The micro-switch used for clicking can fail, and repeats (key bounces) is one of the possible failure modes. I have seen that happen, and replacing the mouse fixed it.
I don’t know if that is your problem, but it is a possibility to consider.
Must be a microsoft mouse rotfl!
It’s a dell. And it’s working fine under windowz and from the usue 11.3 livecd.
Looks like I accidentally actrivated some obscure “control” that is not accessible
from the kde interface…but it sucks…
Do you have access to another mouse to try?
Is it USB?
Connected to the main USB ports?
The mouse that went bad for me was a Dell usb mouse on a Dell computer. I don’t remember whether it misbehaved in Windows.
My wife has a similar Dell mouse for her computer, and she was not using it. So I tried that, and it completely fixed the problem. That’s when I decided that it was faulty switch on the mouse.
You could check the accessibility settings, but it sure sound like a defective mouse switch. Just test with another mouse.
I do have one at the office, usb optical. Light and precise, it’s quite good in fact, except that every half hour or so it jumps to the lower left corner of the screen on it’s own. Perhaps it’s a microsoft “feature”…
I wiped the mouse underside, blew some air on it, same for the
usb connector and seems its back to normal or close to it.
So simple 
Dirty switch, maybe, but if so you’re lucky, as these switches are usually sealed. IME this thing usually only gets worse, I think you’ll end up replacing it soon.