Samsung has today started rolling out the Android v6.0.1 Marshmallow update for the Galaxy A8 Duos (1st Gen.) in India. In addition to all of the new features in Marshmallow included September security patch to the handset.
The update size is around 890MB and should be making its way to the unlocked model of the Galaxy A8 Duoos in the region throughout the day. it may take some time for the update to hit your device. If you did not get this you can check it into the Settings » About device » Software update.