Why are the four QoS steps (classification, queuing, scheduling, and marking/remarking) executed twice for every...

Once before going through each of the redundant switching fabrics.

To protect against failure of either ingress or egress card.

Once before going through the switching fabric, and once more before going onto the egress port.

Once to decide if the packet is in-profile or out-of-profile, and once more to process the packet according to its profile.