ag.source_barcodes_surveys is missing a UNIQUE constraint

...as a result, a `(barcode, survey_id)` can be replicated. There are some examples of this, the reason why is not known.

Locate the schema or migration defining ag.source_barcodes_surveys and inspect how (barcode, survey_id) is represented. Check the reported duplicate examples and determine the project’s expected handling of existing duplicates before enforcing uniqueness. Done means the pair cannot be replicated in the table and the relevant database checks pass.

Written by the indexing model from the issue text.