%IP-4-DUPADDR: Duplicate address 192.168.10.50 on Vlan10, sourced by mac 001a.2b3c.4d5e
In plain English: Two devices are claiming the same IP address.
Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.
What it means
Two devices are claiming the same IP address. The router is warning that the address it owns (or routes for) is being answered by another device with the MAC address named in the message. Traffic for that address now goes to whichever device answered last — so the service on it works, breaks, works, breaks, in a pattern that looks random until you see this log line.
Likely causes, in order
A device was given a static IP that DHCP had already handed to someone else.
A server or printer kept its old static address after the network was renumbered.
A virtual machine was cloned and both copies came online with the same address.
Someone plugged in a home router or test box pre-configured with a common address like .1 or .254.
Check, in order
show arp | include 192.168.10.50Watch whether the MAC address for the IP changes between checks — two different MACs prove two devices.
show mac address-table address 001a.2b3c.4d5eFinds the switch port where the impostor device is plugged in.
The fix
Find the second device through its MAC address and port, then give one of the two a different, free address.
Keep static assignments outside the DHCP pool, and keep a simple record of which addresses are reserved.
Re-check show arp: the address should sit on one stable MAC address and the log should go quiet.
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