64 bytes from 216.18.0.178: icmp_seq=1509 ttl=238 time=79.942 msec (DUP!)

What does DUP mean?
Anyone know

A duplicate packet. The ping program has received the same packet before. This could happen when there is a network confusion where two network interfaces 'have' the same IP address. I've also seen a badly configured Win98 box respond to ping twice.