Relation, authorisation and sequence errorsRelation & missing element
CDS12026Uniqueness Error: The same data element cannot be amended more than once in the same request.
HMRC explanation
What to check across the Relation, authorisation and sequence errors
Two elements, or a value and a rule, disagree. Read both elements together: completing one element with a particular code often requires another value, and the fault is frequently in the element you did not change. The family also covers parties and authorisations that are not recognised, references that must be unique, and messages sent out of sequence.
A declaration that clears CDS can still be wrong
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Source: HMRC, Customs Declaration Service error codes, Open Government Licence v3.0. GOV.UK publication · Licence