Maybe it is a setting, i have searched everywhere, but this is really mysterious
take a linux, configure this kind
eth0: 192.168.10.200 / 255.255.255.0
eth1: 192.168.10.201 / 255.255.255.0
Both Interfaces are "real ethernet cards" and are connected to same switch
if you take now a 3'rd machine, and do a
arping -c 1 192.168.10.20060 bytes from 00:0c:29:bc:96:fe ( 192.168.10.200): index=0 time=645.876 usec
60 bytes from 00:0c:29:bc:96:f4 ( 192.168.10.200): index=1 time=1.472 msec
arping -c 1 192.168.10.20160 bytes from 00:0c:29:bc:96:fe (192.168.10.201 ): index=0 time=833.988 usec
60 bytes from 00:0c:29:bc:96:f4 (192.168.10.201): index=1 time=1.211 msec