Duplicate line remover

This tool takes a list of lines and returns exactly one copy of each: the first occurrence retained, every subsequent exact repeat discarded. The relative order of the surviving lines is unchanged from the input, so the position of a unique line in the output matches where it first appeared going in.

Deduplication runs client-side, in JavaScript, entirely within your browser. There is no upload and no server round trip, and therefore no run limit.

No configuration required.

Step 1

Paste the list, one item per line, into the input field.

Step 2

Run the deduplication. It executes instantly, locally.

Step 3

Read the deduplicated result.

The algorithm traverses the input from the first line to the last, retaining the first instance of each distinct line and discarding every later exact repeat. The result is not reordered by any criterion; it reflects the original sequence of the input with duplicates removed.

This distinction is material whenever sequence carries information: a chronological log, an ordered sequence of steps, or a list where the earliest mention is the authoritative one. A tool that sorted before deduplicating would produce a correct set of unique values in a sequence that no longer corresponds to the source.

Two lines are treated as duplicates only when they are identical character for character. 'Apples' and 'apples' compare as distinct lines under this rule, since the differing capitalization produces two different strings, regardless of whether a human reader would consider them equivalent.

For case-insensitive deduplication, normalize the text to a single case with the case converter first, then deduplicate the result. This ordering matters: converting case after deduplication will not catch repeats the case difference had been masking.

This is the most frequent cause of a duplicate surviving the pass. 'apples' and 'apples ', the second carrying a trailing space, render identically but are unequal strings, so an exact-match comparison treats them as two separate lines.

The same applies to a trailing tab, or a stray carriage return left over from a Windows-to-Unix line-ending conversion. If duplicates persist through this tool, run the whitespace minifier or a hidden-character cleaner first to normalize whatever is making visually identical lines byte-different underneath, then run the deduplication a second time, since normalizing whitespace can itself turn two previously distinct lines into exact matches.

A mailing list carrying a repeated address sends the same message to the same recipient twice. A CSV import with duplicate rows inflates a figure meant to represent unique entries. A configuration file with a setting repeated across two lines can behave unpredictably, depending entirely on which line a given parser reads last. A list of URLs to crawl, fetch or check with duplicates left in wastes exactly as much time and bandwidth as the number of extra copies it carries.

In any automated pipeline consuming a word list, a keyword list or a set of test cases, duplicates cost processing time at minimum and silently double-weight an entry at worst.

Data and marketing teams deduplicate an email list or a customer export ahead of a campaign platform to ensure one entry per unique value. Engineers deduplicate log lines, file path lists and test case sets for the same underlying reason, since a duplicated test case wastes a CI run without adding any real coverage.

It is also used to merge two overlapping lists, combining subscriber exports from separate sources into exactly one copy of every address appearing in either.