01:15:58.747749 < Client1.domain.test > Server.domain.test: icmp: echo
request (DF) 
01:15:58.747808 > Server.domain.test > Client1.domain.test: icmp: echo
reply
01:16:13.587872 < Client1.domain.test > Server.domain.test: icmp: echo
request (DF) 
01:16:13.587936 > Server.domain.test > Client1.domain.test: icmp: echo
reply
01:16:16.598712 < Client1.domain.test > Client2.domain.test: icmp: echo
request (DF)
01:16:16.599321 < Client1.domain.test > Client2.domain.test: icmp: echo
request
01:16:16.631662 > Client2.domain.test > Client1.domain.test: icmp: echo
reply
01:16:16.663876 > Client2.domain.test > Client1.domain.test: icmp: echo
reply
01:16:18.579882 > arp who-has Client1.domain.test tell Server.domain.test
(x:xx:xx:xx:xx:xx)
01:16:18.585470 < arp reply Client1.domain.test is-at 0:40:96:30:d5:46
(x:xx:xx:xx:xx:xx)
01:16:22.677906 < Client1.domain.test > Server.domain.test: icmp: echo
request (DF)
01:16:22.677972 > Server.domain.test > Client1.domain.test: icmp: echo
reply

The echo request from Client1 were for some reason replied to with a source
address of another client (Client2) instead of the Server.

I just noticed this debugging the ntpd problem previously reported, so I'm
not sure if it's a kernel or tcpdump problem ... I suspect kernel since
tcp dump didn't have any reason to resolve Client2's IP or name otherwise.