Clean old installs:
apt-get remove --purge fglrx*
sync
reboot
Install new driver (use defaults when prompted):
chmod 755 amd-driver-installer-12-4-x86.x86_64.run
chmod +x amd-driver-installer-12-4-x86.x86_64.run
./amd-driver-installer-12-4-x86.x86_64.run
aticonfig -f --initial --adapter=all
sync
reboot
Check cgminer is seeing all the devices:
export DISPLAY=:0
export GPU_USE_SYNC_OBJECTS=1
./cgminer --ndevs
I would also check top to make sure only one instance of cgminer is launching.