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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
- RFC 2181: RRset definition and data consistency
- Microsoft: DnsQuery_W record lists
- Microsoft: DNS system error codes
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