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Line Deduplicator

Remove duplicate lines while preserving order

About Line Deduplicator

Duplicate lines are a surprisingly common problem: merging two lists produces repeated entries, exporting data from a spreadsheet or database creates duplicate rows, or copy-pasting from multiple sources adds the same item multiple times. Removing them manually from a list of hundreds is tedious and error-prone. Line Deduplicator removes every duplicate line from your text in a single operation, keeping only the first occurrence of each unique line by default. It also supports keeping the last occurrence, and optionally normalizes case or trims whitespace before comparing lines — so 'Apple', 'apple', and ' apple ' can all be treated as the same entry. The original line order is preserved so your deduplicated list stays in the sequence you expect.

Why use Line Deduplicator

Order-Preserving Deduplication

Removes duplicates while keeping your lines in their original sequence — not sorted or shuffled.

Case-Insensitive Matching

Treat 'NYC' and 'nyc' as the same entry so mixed-case duplicates are caught.

Whitespace-Tolerant Comparison

Ignore leading and trailing spaces so ' item ' and 'item' are recognized as duplicates.

First or Last Occurrence Control

Keep the first or last instance of each duplicate to control which version survives.

Duplicate Count Display

Shows how many lines were removed so you can verify the operation completed correctly.

Handles Large Lists Instantly

Deduplicates thousands of lines in milliseconds entirely in the browser.

How to use Line Deduplicator

  1. Paste your text with duplicate lines into the input area.
  2. Choose whether to keep the first or last occurrence of each duplicate.
  3. Toggle 'Case insensitive' if you want 'Apple' and 'apple' to count as the same line.
  4. Toggle 'Trim whitespace' to ignore leading/trailing spaces when comparing.
  5. The deduplicated output appears instantly in the output panel.
  6. Click Copy to copy the clean, deduplicated list to your clipboard.

When to use Line Deduplicator

  • When merging two or more lists and needing to remove items that appear in multiple source lists.
  • When cleaning up a CSV export that contains duplicate rows from a database query.
  • When deduplicating a keyword list before uploading to an ad platform.
  • When combining autocomplete suggestions or log entries that repeat frequently.
  • When cleaning up a list of email addresses or domain names before sending a batch.
  • When normalizing a tag or category list where the same tag was entered with different capitalization.

Examples

Simple list deduplication

Input: apple banana apple cherry banana

Output: apple banana cherry

Case-insensitive deduplication

Input: New York new york NEW YORK London

Output: New York London (case-insensitive mode removes the lowercased variants)

CSV row deduplication

Input: id,name 1,Alice 2,Bob 1,Alice 3,Carol

Output: id,name 1,Alice 2,Bob 3,Carol

Tips

  • Enable both case-insensitive and trim-whitespace toggles when cleaning real-world data where human entry inconsistencies are common.
  • Use 'keep last occurrence' when your list is chronologically ordered and the most recent entry is the most correct version.
  • After deduplication, run Alphabetizer to sort the clean list if order no longer matters.
  • Paste the output into Word Frequency Counter to verify no term still appears multiple times after deduplication.
  • For domain or email lists, case-insensitive deduplication is essential since 'User@Example.com' and 'user@example.com' are the same address.

Frequently Asked Questions

Does it preserve the order of the original lines?▾
Yes. The first (or last, depending on your setting) occurrence of each unique line is kept in its original position. The output is not sorted — it maintains the input sequence with duplicates removed.
What counts as a duplicate line?▾
By default, two lines are duplicates if they are exactly identical including case and whitespace. Enable case-insensitive mode and/or trim whitespace to broaden what is considered a match.
Can I keep the last occurrence of a duplicate instead of the first?▾
Yes. Toggle the 'Keep last occurrence' option and the tool will retain the final appearance of each duplicate line and remove the earlier occurrences.
Does it count blank lines as duplicates?▾
Yes. Multiple consecutive blank lines are treated as duplicate empty lines and reduced to a single blank line (or removed entirely depending on your blank line handling setting).
Is there a limit on how many lines it can process?▾
There is no enforced limit. The tool processes tens of thousands of lines in milliseconds using a JavaScript Set for O(n) deduplication.
Does it work on CSV data?▾
It works on entire rows as strings. If two CSV rows are byte-for-byte identical (including all commas and values), they will be deduplicated. It does not compare individual CSV fields.
Can I use it to find which lines were duplicated?▾
The tool shows the count of removed lines. To see which specific lines were duplicates, compare the original and output using the Text Diff tool.
Does case-insensitive mode change the capitalization of the output?▾
No. Case-insensitive mode only affects comparison — the surviving line's original capitalization is preserved as-is in the output.

Explore the category

Text Reverser Reverse text, lines, or characters per line Random Line Picker Pick one or many random lines from text Line Numberer Add line numbers to text with custom start, padding, separator, and empty-line handling. Text Diff Compare two texts line-by-line Text Cleaner Remove extra spaces, symbols and more Alphabetizer Sort text lines alphabetically, reverse, or by length
How to Read and Write JSONL (JSON Lines) — The Streaming Format JSONL puts one JSON object per line, making it streamable, appendable, and easy to process without loading the entire file into memory. Here Mar 2026 · 7 min

Glossary

Deduplication
The process of removing duplicate entries from a dataset, keeping only one instance of each unique value.
Set
A data structure that stores unique values only. JavaScript's Set is used internally for O(n) deduplication without nested loops.
Case-insensitive comparison
A string comparison that treats uppercase and lowercase letters as equivalent, so 'Apple' and 'apple' are considered the same string.
First occurrence
The earliest appearance of a value in the input sequence. First-occurrence deduplication keeps this and removes all subsequent duplicates.
Last occurrence
The latest appearance of a value in the input sequence. Last-occurrence deduplication keeps this version and removes all earlier duplicates.
Whitespace trimming
Removing leading and trailing whitespace (spaces, tabs) from a string before comparing it, so padded variants match their unpadded counterparts.