Join two CSV files by ID without multiplying rows

Step-by-step approach

Check both CSV files have a key column with consistent identifier formatting.
Ensure the right-hand file has unique key values.
Run CSV Join by Key and select both files.
Investigate unmatched keys and renamed columns in the output.
Use a copy of the merged CSV for downstream imports.

Try the matching tool

Open CSV Join by Key to work on a local copy. Review the result before importing it elsewhere.

What this does not solve

Browser tools can validate known structures and transform available data, but cannot restore deleted or corrupted bytes, infer unknown business facts, or guarantee that a target system will import every field as intended.