Paste a CSV and repeated rows are removed. By default the whole row has to match; name one or more columns and only those are compared, which is how you deduplicate a mailing list on the email address while the rest of the row differs. The copy that stays is the first one, written exactly as it arrived, and the summary states how many rows went in, how many came out and how many were dropped — so the arithmetic is yours to check.
This page loads no libraries at all — the work is done by the browser itself. Your file is read on your own machine and never sent anywhere — open your browser's network tab and watch while you use it.
Yes. Type the column heading — or several separated by commas — and only those fields are compared. That is the usual case: two exports of a contact list disagree on a timestamp or a note field, so comparing whole rows finds no duplicates at all while the email address is clearly repeated.
You are told that no such column exists, and shown the ones that do. Ignoring an unknown name would be far worse: the tool would quietly fall back to comparing whole rows, report that nothing was duplicated, and leave you believing the list was clean.
The first, byte for byte as it appeared. Comparison ignores case and surrounding spaces by default, but nothing is rewritten — a deduplicator that lower-cases your data while removing repeats has edited the file it was asked to tidy.
The summary gives three numbers: rows in, rows out, and duplicates removed. If they do not add up, something is wrong and you can see it immediately rather than trusting a green tick.
Everything here is free and works the same way — in your browser.