I have been having some issues with our Cisco 2901 including this log I keep getting that says "%IP-4-DUPADDR: Duplicate address x.x.x.x on GigabitEthernet0/0.xxx, sourced by <firewall-mac-address>". This should be as simple as tracking down the interface on the firewall with the duplicate address, but it does not exist. I have done a 'show interfaces all' command and there is no duplicate address on the firewall. There has been a few other issues with the router lately that I am having a hard time figuring out, including a bunch of overflow errors despite there not being a lot of traffic and high CPU usage. Any idea what would cause this duplicate ip error when there isn't a duplicate IP?
As the device in question is a firewall I wonder if the mac-address in question is a virtual mac-address, which is why you can't correlate this to a mac-address on a physical interface? This virtual mac-address can be shared between redundant firewalls, which may explain the log entry. In which case this would not strictly be a problem.
I would check how virtual mac-addresses are handled by your particular make and model as a first step.