Does re-running an import duplicate my products?

No, provided you’ve configured a unique identifier that’s stable between runs (typically SKU). The match-existing stage finds the existing posts and routes them to UPDATE rather than INSERT.

If you see duplicates after a re-import, the most likely cause is the unique identifier changing between runs (e.g. you re-mapped SKU to a different column). See Choosing your unique identifier for the constraints.

Two rows with the same identifier in one file stop the import before anything is written, so a file cannot create two copies of one item. Identifiers are compared in full, however long they are. To find duplicates that are on your site, use the Data tab checks for duplicate SKUs and titles.

Further reading

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Written for Super Speedy Imports 3.40.0.