Pinging some hosts causes multiple responses for a ping echo request. This is reported by the Linux ping command but not by OSX or Windows. Typically it appears as:
290cottrell@pinger:~$ping www.cern.ch
PING webrlb02.cern.ch (188.184.9.235) 56(84) bytes of data.
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=1 ttl=109 time=163 ms
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=1 ttl=109 time=163 ms (DUP!)
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=1 ttl=109 time=163 ms (DUP!)
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=2 ttl=109 time=163 ms
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=2 ttl=109 time=163 ms (DUP!)
64 bytes from webrlb02.cern.ch (188.184.9.235): icmp_seq=2 ttl=109 time=163 ms (DUP!)
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
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