Windows error 9555 (DNS_ERROR_NOT_UNIQUE): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The queried DNS owner name and record type produced more than one record where one was required.

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DNS_ERROR_TRY_AGAIN_LATER	DNS_ERROR_NON_RFC_NAME

DNS_ERROR_NOT_UNIQUE

DNS RRsets can legitimately contain multiple records

DNS_ERROR_NOT_UNIQUE means that the operation expected one record for a name and type but found a non-unique result. DNS itself permits an RRset to contain multiple records with the same owner, class, and type. Multiple A or AAAA addresses, MX exchanges, NS servers, and SRV targets are common designs rather than database corruption.

The error therefore exposes a mismatch between the operation’s uniqueness requirement and the zone data. Selecting the first returned record is unsafe because DNS record order is not a durable identity rule and can change across responses, servers, or load-balancing decisions.

What to inspect

Enumerate the complete returned list and group records by owner name, class, and type.
Determine why the caller requires one value: configuration identity, reverse mapping, update prerequisite, or an application-specific constraint.
Remove duplicates only when zone policy requires uniqueness; otherwise update the application to process the RRset correctly.
For SRV or MX data, honor their protocol-defined priority and selection fields instead of arbitrary list order.

References

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