This is almost certainly a cosmetic bug. We have seen it with Polycom phones as well as a few other devices.
First verify that there is indeed no actual duplex issue on the port. Look at the output of "show interface" for incrementing CRC errors, collisions, late collisions and runts. Log on to the Windows box on the other side of the link and verify that it shows the same speed and duplex that the switch is reporting.
You can either turn off LLDP completely on the port or suppress the mis-reported data with the interface-level command no lldp receive 802.3-info mac-phy-config .