On my Asus laptop K53U, /usr/lib/gnome-settings-daemon/gsd-backlight-helper adjusts /sys/class/backlight/acpi_video0 when receiving KEY_BRIGHTNESS-UP/-DOWN while the BIOS in fact adjusts the brightness itself on the key pressed too.
This causes two levels of backlight brightness change on one key press.
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ApportVersion: 1.23-0ubuntu3
Architecture: amd64
DistroRelease: Ubuntu 11.10
InstallationMedia: Ubuntu 11.10 "Oneiric" - Build amd64 LIVE Binary 20111028-04:06
Package: gnome-settings-daemon 3.2.1-0ubuntu3
PackageArchitecture: amd64
ProcEnviron:
LANGUAGE=en_US:
PATH=(custom, no user)
LANG=en_US.UTF-8
SHELL=/bin/bash
ProcVersionSignature: Ubuntu 3.0.0-12.20-generic 3.0.4
Tags: oneiric running-unity
Uname: Linux 3.0.0-12-generic x86_64
UpgradeStatus: No upgrade log present (probably fresh install)
UserGroups: adm admin cdrom dialout lpadmin plugdev sambashare
apport information