The detection of devices with duplicate media access controller (MAC) addresses in a cable network. A cable modem (CM) having a MAC address is connected by the cable network to a cable modem termination system (CMTS) having a gateway interface address. A centralized storage of historical cable modem MAC address/giaddr pair data is used to identify cable modems (CM's) that report duplicate MAC addresses. A server tracks the CM MAC address/giaddr pair data of all CM requests that it receives and stores the MAC address/giaddr pair data into a datastore (such as a database). When a CM seeks to access the network, the server looks into the datastore to determine whether the CM MAC address of the CM has previously been stored with a different associated giaddr, which would imply that there are multiple CMs attached to different CMTSs where the CMs share the same MAC address. If such duplication is detected, a appropriate remedial response is taken.