DUP significa pacote duplicado.
De 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).
Existem diferentes razões para isso: você capturou seu tráfego de rede com uma interface em modo promíscuo? Às vezes, essa é a razão para pacotes dupplicados.