Remove Duplicate Lines

Lists collect duplicates. Email addresses exported from two places, keywords gathered from several reports, URLs pasted from a dozen tabs — all of them end up with the same entry more than once, and spotting repeats by eye in a list of several hundred is hopeless.

This tool keeps the first time a line appears and removes every later copy. Crucially, it does not sort: the order you pasted is the order you get back. That matters for anything where position carries meaning — a ranked list, a sequence of steps, a log — and it is the main way this differs from piping text through a sort-and-unique command.

Two options change what counts as a duplicate. Ignoring surrounding spaces is on by default, because a trailing space is invisible and would otherwise make two identical entries look different. Matching case exactly is off by default, so Apple and apple are treated as the same entry; turn it on when case is significant, as in a list of identifiers or passwords.

You are told how many lines went in, how many came out, and how many were removed, so you can sanity-check the result rather than trusting it blindly. Everything runs in the page, so a list of customer emails is never uploaded anywhere.