This message indicates that your Catalyst 4000 switch has learned a MAC address 00:00:5e:00:01:65 that already exists in its CAM table on a port other than the original one, which means that this host is dually attached to the switch.
What you can to track down how this MAC address was learned from both ports.
Step 1: Consider port the first port. Issue the show cam dynamic [port] command. If you see the MAC address 00:00:5e:00:01:65 coming
out in the list of the MAC addresses learned on this port, try to see if this is a single host that is connected or if there are multiple hosts registered on that port.
Step 2: If it is a single host that is connected, check the other port that is registered and see if the host is dually attached to the switch. If however, the host has connections to other devices that can eventually lead back into this switch, try to track down the intermediate devices that are in between.
The best way to achieve this (assuming that they are Cisco devices) is to use the
output of the command show cdp neighbors detail.
