Hello,
Are You sure You are NOT pinging the broadcast IP? DUP! happens when a subnet broadcast IP, such as 192.168.0.255 for 182.168.0.0/24 subnet, is pinged.
Assuming You are not pinging broadcast IP, then I'd suggest to establish first whether the DUP! really means two Echo reply packets, or this is a display issue.
If You do Your pinging from the switch with "detail" knob, You'd see the incoming interface into which the reply arrives:
regress@labrouter> ping 10.254.55.2 detail PING 10.254.55.2 (10.254.55.2): 56 data bytes 64 bytes from 10.254.55.2 via ae0.577: icmp_seq=0 ttl=64 time=11.081 ms ^C --- 10.254.55.2 ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max/stddev = 11.081/11.081/11.081/0.000 ms
If You see two different "VIA" interfaces, then there is either a packet duplication or a loop in Your network.
Then You can capture the packets with "monitor traffic interface <blah> size 9999 extensive protocol icmp" and see what MAC/TTL/other fields are in there to arrive at conclusions.
If You see same "VIA" interface for the 1st and 2nd packet then there is a packet duplication somewhere between switch You are pining from, and ping destination.
HTH
Thx
Alex