A worked example
Paste a sign-up list of 500 addresses exported from two sources. With the defaults, "Alice@Example.com " and "alice@example.com" collapse into one entry, and the report tells you which addresses appeared twice — which is usually more interesting than the clean list, because it tells you how much the two sources overlap.
Why case matters more than it looks
The local part of an email address is technically case-sensitive, but in practice no mail provider treats it that way — so two rows that differ only in capitals are one person and one unsubscribe. Identifiers, file paths on Linux and base64 strings are the opposite: there, a capital is a different value, and "Match case" is the setting you want.
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Frequently asked questions
Is this duplicate remover free?
Yes, with no account and no limit on how many lines you paste.
Does it ignore capitals?
By default, yes — "Alice@example.com" and "alice@example.com" are one address, and treating them as two is the usual reason a de-duplicated mailing list still sends twice. Turn on "Match case" when the difference matters, as it does for codes and identifiers.
Which copy is kept?
The first one, in the order you pasted them. The rest of the list keeps its original order too, so a list that was already sorted stays sorted.
Can I see what was repeated rather than remove it?
Yes — "Keep only duplicates" returns just the entries that appeared more than once, which is how you find the double-booked names in a sign-up list. "Keep only unique" does the opposite and returns entries that appeared exactly once.
Does trailing whitespace count as a difference?
Not while "Ignore surrounding spaces" is on, which it is by default. A column copied out of a spreadsheet often carries trailing spaces on some rows and not others, and without trimming those look like different values.
Is my list uploaded?
No. De-duplication happens in your browser, so a customer list or a set of internal URLs never leaves your device.