If multiple rows in the CSV file contain the same record ID in the ID column, those rows are considered to be duplicates. As a best practice, we recommend that you review your CSV file prior to update to remove any duplicate rows.
If the Upsert or Update operation attempts to process duplicate entries in your CSV file within the same batch of records, the operation will fail with 'Duplicate Id Specified' and specify the duplicate Id.
Error: Duplicate id specified: 0013000000auCyTAAU
Where 0013000000auCyTAAU is the Id that's contained in multiple rows within your CSV.
However, if the duplicate rows are processed as a part of separate or different batches, the corresponding record in Salesforce is updated with the values each row, and will reflect the values in the last rows when the update has completed.
Example:
000384876

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