Duplicate external id specified [DUPLICATE_EXTERNAL_ID]

When doing an Upsert / Update on multiple records, and the Id values provided are in the data multiple times, then Salesforce will reject the records. This is to prevent multiple updates / inserts happening on the same records at the same time.Make sure that there are no Duplicate Ids or External Ids in the source data, as these need to be unique.

For example, here is an example of this type of error:

SF_ERROR: DUPLICATE_EXTERNAL_ID OBJ: Product2 - Case SKU Number (US): more than one record found for external id field: [01tj00000033KNPAA2, 01tj0000002xwceAAA] FIELDS: Field_Source__C.

This is a Salesforce restriction. Salesforce will not allow multiple records with the same External ID to be created. So the records with duplicate external IDs need to be checked and adjusted accordingly.