I've run in to this on 3750G's in a various sized stacks. We apply port security for a mac address on a single port (not existing on more than one port - that's a different issue that appears in multiple posts already).
In this case:
1) We do a 'sho mac address-table and see that the device with the mac address in question lives on one port, port 1/0/x.
2) We apply mac port security for this exact same mac address to the same port it is already attached to.
switch(config-if)#switchport port-security mac-address 001a.1ec8.abcd
3) Get this error: 'Found duplicate mac-address 001a.1ec8.abcd'
We again confirm that that is the only port on the entire switch that has this mac-address.
Try the command again, same error.
4) We do a bunch of show commands, get in and out of the switch, go back and then try it again, and now it works, same command, same port and same mac address.
...Aging is default 300
...These are Cisco wireless AP's attached to the switch
...This occurs on different switches with different ports using different mac addresses, always same symptoms.
Before I did in to a long drawn out isolation effort, anybody seen this before?