Duplicate Line Remover

Duplicate lines accumulate wherever data gets merged. Concatenated CSV exports repeat their header row, mailing lists collected from several sources overlap heavily, and log files re-emit the same event. Deduplicating before you count, import, or analyse anything is the difference between a number you can trust and one quietly inflated by however much overlap happened to exist.

This keeps the first occurrence of each line and drops the rest, which preserves the original order rather than sorting the output. That matters when sequence carries meaning — a transcript, a set of steps, a chronological log — because a dedupe that also reorders destroys information the list depended on. It does mean that if two identical lines differ in some way you care about, the earlier one wins.

The important limitation is that matching is exact. "john@example.com" and "John@Example.com" are the same mailbox and will both survive, as will two lines differing only by a trailing space or a non-breaking space pasted in from elsewhere. Normalising case and whitespace first catches substantially more real duplicates. And check the output before discarding the original: repeated lines are deliberate in poetry, in refrains, and in legal documents where a clause is restated by design.