What could DUP mean when using ping?

DUP means duplicate packet.

From 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).

There are different reasons for this, did you capture your network traffic with an interface in promiscous mode? Sometimes this is the reason for dupplicated packets.

It means the reply packet is a duplicate. ICMP responses contain a sequence number. Sometimes you'll actually get more than one reply with the same sequence number. This is usually caused by faulty networks. Wireless networks often rely on re-sending packets if they don't get proper verification, and if the first one actually worked, the re-send will be a duplicate. In normal usage duplicates are discarded. In ping replies they are noted because they tell you something about how the network is functioning. For example, if you are getting 100% duplicates some fault detection mechanism is likely broken.

Check out if couple machines aren't using same IP. It was my case for that message.

I run into this when I cloned a vm machine and run both at the same time. Network configuration was in a bridge mode and IP linked to mac. So when second machine start using network I experienced problems with ssh:

write: Connection reset by peer
ssh: connect to host 172.16.13.105 port 22: Connection refused
ssh_exchange_identification: read: Connection reset by peer

And the ping error when ping.