Comment 11 for bug 286339

I'm experiencing the same behavior (each event registered twice) with a panasonic laptop, due to the new panasonic_laptop kernel module. Issue is resolved similarly for panasonic laptops by removing /etc/acpi/events/panasonic-[mute,volume-down,volume-up]

Note that the kernel module isn't automatically loaded, at least for me, so that should be resolved before applying this fix.