I recently SOLVED this issue, sorry if I didn't wrote an update here.
The trick (I'm not completely sure) was going to KDE System Settings >
"Keyboard and mouse", and playing a bit with "Enable keyboard repeat" in the
Keyboard page.
Maybe I just activated then deactivated it, then my problems with multimedia
keys are gone.
As I told you, I'm not completely sure on what resolved the multiple key
presses issue, but I was playing with settings in that page and suddenly the
issue is gone (also after rebooting the system).
I recently SOLVED this issue, sorry if I didn't wrote an update here.
The trick (I'm not completely sure) was going to KDE System Settings >
"Keyboard and mouse", and playing a bit with "Enable keyboard repeat" in the
Keyboard page.
Maybe I just activated then deactivated it, then my problems with multimedia
keys are gone.
As I told you, I'm not completely sure on what resolved the multiple key
presses issue, but I was playing with settings in that page and suddenly the
issue is gone (also after rebooting the system).