Comment 34 for bug 310270

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 suddendly the issue is gone (also after rebooting the system).

IMHO this strange issue is not (only) in Xorg as you suggested, because playing with a KDE configuration solved it.