Which occurrence of a duplicate line is kept?

The first one. Later lines that exactly match an earlier line are dropped, and the surviving lines keep their original relative order.

Is the comparison case-sensitive?

Yes, lines are compared as exact strings, so 'Apple' and 'apple' are treated as different lines and both are kept.

Does it trim whitespace before comparing?

No, lines are compared exactly as typed, so trailing spaces or differing indentation make otherwise-identical lines count as distinct.

Duplicate Line Remover Online - Deduplicate Text Lines

Overview

Introduction

Pasted lists, log exports, and CSV-like data often end up with repeated lines after merging multiple sources, and picking out the duplicates by eye is slow and error-prone.

This tool strips them out automatically while keeping the list in its original order.

What Is Duplicate Line Remover?

A line deduplicator that scans newline-separated text and removes every repeated line after its first appearance, leaving one copy of each distinct line.

It runs entirely client-side as part of this site's String Tools collection, so nothing you paste is ever uploaded to a server.

How Duplicate Line Remover Works

The input is split into lines, and each line is checked against a running set of lines already seen; a line is kept only the first time it appears and dropped on every later repeat.

The transformation happens synchronously in JavaScript the moment you type, with no network round trip involved.

When To Use Duplicate Line Remover

Use it after merging two lists, exporting a report, or copying data from multiple sources that might overlap.

It's a fast way to get a clean, deduplicated list without opening a spreadsheet or writing a one-off script just to check.

Often used alongside Text Sorter and Empty Line Remover.

Features

Advantages

Preserves the original order of the surviving lines instead of resorting them.
Exact-match comparison is fast and predictable for large pasted lists.
Tracks seen lines in a Set, so the work stays linear in the number of lines rather than degrading as the list grows.

Limitations

Comparison is case-sensitive and whitespace-sensitive, so near-duplicates that differ only in case or trailing spaces are not merged.
Always keeps the first occurrence, so there is no option to retain the last one instead when later entries are the more current.

Examples

Removing repeated lines

Input

apple
banana
apple
cherry
banana

Output

apple
banana
cherry

Each line's first occurrence is kept; the later repeats of 'apple' and 'banana' are dropped.

Best Practices & Notes

Best Practices

Run Trim String or a case converter first if your list has inconsistent spacing or casing you want treated as duplicates.
Arrange the list so the entry you want to keep appears first, because only the earliest occurrence of each line survives.
Compare the line count before and after to confirm how many duplicates were actually removed.

Developer Notes

The implementation walks the lines once, tracking seen values in a `Set<string>` for O(1) membership checks, and only pushes a line to the output array the first time it's encountered.

Duplicate Line Remover Use Cases

Cleaning up a list merged from multiple sources
Deduplicating exported log lines
Removing repeated entries from a pasted CSV column

Common Mistakes

Expecting case-insensitive or whitespace-insensitive matching; comparison is exact.
Overlooking invisible trailing whitespace, which makes two otherwise identical lines compare as different so both are kept.

Tips

Pair with Text Sorter afterward if you also want the deduplicated list alphabetized.
Blank lines are treated as ordinary lines, so a run of repeated blank separators collapses to a single blank line as well.

Duplicate Line Remover