DUP means duplicate packet.
From `man ping`:
> Duplicate and Damaged Packets
>
> ping will report duplicate and damaged packets. Duplicate packets should never occur, and seem to be caused by inappropriate link-level retransmissions. Duplicates may occur in many situations and are rarely (if ever) a good sign, although the presence of low levels of duplicates may not always be cause for alarm.
>
> Damaged packets are obviously serious cause for alarm and often indicate broken hardware somewhere in the ping packet's path (in the network or in the hosts).
There are different reasons for this, did you capture your network traffic with an interface in promiscous mode? Sometimes this is the reason for dupplicated packets.