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ERROR_DUP_DOMAINNAME
The requested workgroup or domain name conflicts with an existing network name.
ERROR_DUP_DOMAINNAME has decimal value 1221 (0x4C5). Windows detected that the proposed workgroup or domain name is already in use by another computer or network identity. This is a name-collision result, not a malformed-domain result.
Where collisions arise
- a computer is assigned a workgroup name that another machine is advertising as its computer name
- cloned systems start with duplicate identity or stale network configuration
- a NetBIOS compatibility name conflicts even though DNS names differ
- an old machine remains online during a rename or migration
- cached browser or name-service data temporarily reports a retired owner
Evidence to collect before renaming anything
Record the proposed DNS and NetBIOS forms, local computer name, domain membership, network interfaces, and the responder that claims the conflicting name. Check DNS, WINS or NetBIOS name data where used, directory computer accounts, and packet traces from the collision interval. A single ping reply is not enough to identify ownership.
Resolution sequence
Determine which system legitimately owns the name. Shut down or rename unintended clones, clean stale directory and name-service records according to administrative policy, and then repeat validation. If both systems are legitimate, choose a new unique name rather than forcing registration and creating intermittent authentication or discovery failures.
Domain migration considerations
- coordinate DNS and NetBIOS namespace choices
- verify trusts, SPNs, and automation that embeds the old name
- avoid reusing retired names until replication and caches have converged
- check rollback plans before renaming domain infrastructure
- audit which controller or provisioning job assigned the duplicate
Difference from related codes
ERROR_INVALID_DOMAINNAME means the name syntax is invalid. ERROR_DUP_NAME is another duplicate-network-name result used in different contexts. Error 1221 specifically describes a workgroup or domain name already used by another computer.
Retry policy
Immediate retries can amplify a name-registration conflict. Retry only after the conflicting owner or stale registration has been resolved and allow time for directory or name-service propagation. Keep the original collision evidence because the competing machine may disappear before investigation.
Example
A lab image is cloned with the computer name BUILD01, while an automation field also sets the workgroup to BUILD01. When the clone joins the segment, Windows returns 1221. Inventory and packet data identify the existing owner, and the image pipeline begins assigning unique computer and workgroup values before network activation.
References
- Microsoft: System Error Codes (1000–1299)
- Microsoft: Naming conventions for computers and domains
- Microsoft: Naming conventions
- Microsoft: Service Principal Names
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