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Duplicate packets should never occur when pinging a unicast address, 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. Duplicates are expected when pinging a broadcast or multicast address, since they are not really duplicates but replies from different hosts to the same request. From http://www.gsp.com/cgi-bin/man.cgi?section=8&topic=ping#4
For a discussion of the reasons for duplicate pings see http://unix.stackexchange.com/questions/13254/what-could-dup-mean-when-using-ping. For PingER data several of the possibilities such as multicast, wireless network, promiscuous mode.
Duplicate ping responses can be seen for example from SLAC to CERN or www.realbroadband.co.sz. They can be caused by:
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