Columnwise Open the workspace

Free CSV tool

How to find duplicate rows in a CSV by key column

Load one file, pick the column that should be unique per row — a patient ID, order number, email address — and Columnwise tells you exactly how many rows break that rule, with an example value, before you build anything else on top of the file.

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Runs entirely in your browser — your file stays on your machine.

How it works

  1. Load your file. Just one — drag in a CSV/TSV/XLSX file and Columnwise checks it against itself.
  2. The check is already set up for you — a duplicate-key step is added automatically, pointed at the file you just loaded on both sides.
  3. Pick the key column(s) that should be unique per row — e.g. a patient ID, order number or email address.
  4. Read the report. If any key value repeats, you get the exact duplicate count and an example value, with no further setup required.

Worked example

Checking a patient list for duplicate IDs

patients.csv is supposed to have exactly one row per patient_id. Before merging anything else onto it, pick patient_id as the key and assert that it's unique.

If a data-entry duplicate snuck in — the same patient registered twice under the same ID — you'll see a message like:

You asserted one-to-one but the left key has 1 duplicate value (e.g. "PT-00482")
— this join will produce 1 extra row.

That's one row you can go check by hand, instead of a duplicate silently doubling that patient's visit count in every summary you compute afterwards.

Your file never leaves your computer

The file is read with the browser's File API and checked in memory, in your tab. There's no upload step — open your browser's Network tab while you do this and you won't see a single request carrying the file's contents (the only network activity you might see at all is a Google Analytics page-view ping, if you've accepted analytics — never your data). That's the same architecture the whole free toolkit uses, not a special case for this page.

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No signup, no upload — the workspace opens with a duplicate-key check ready.

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