Remove Duplicate Lines
Clean text by removing duplicate lines, keeping first or last occurrence.
About the Remove Duplicate Lines
The Remove Duplicate Lines tool cleans up lists that have accumulated redundant entries. Paste a column of email addresses, a log file with repeated warnings, a CSV with duplicate rows, or a keyword list with accidental repeats — the tool strips the duplicates and leaves you with a clean, unique set. You can optionally sort the output alphabetically, ignore case when comparing, trim whitespace before comparing, and remove blank lines.
Duplicate data creeps in everywhere. Mailing list exports often contain the same address twice because a subscriber signed up with two different casings. Server logs repeat the same warning dozens of times per minute. Keyword research tools export the same term under different match types. Without deduplication, you end up sending duplicate emails, inflating your line counts, or polluting your keyword strategy with phantom volume.
The tool preserves the first occurrence of each line and removes subsequent copies. This means the original order is kept by default — useful when the order of your data matters (as it does in server logs or sequential records). If you want alphabetical output instead, check the sort option.
How It Works
The deduplication uses a JavaScript Set, which is the native data structure for uniqueness. Each line is processed in order: its key (the line itself, or its lowercased version if case-insensitive mode is on, or its trimmed version if trim mode is on) is checked against the set. If the key is not in the set, the line is kept and the key is added. If the key is already in the set, the line is discarded.
Set lookups are O(1) on average (hash table), so the total algorithm runs in O(n) time — linear in the number of lines. This means the tool handles tens of thousands of lines without noticeable lag. The browser's V8 JavaScript engine optimizes string hashing, so even long lines are compared efficiently.
If the sort option is enabled, the unique lines are sorted using the default Array.prototype.sort(), which performs a lexicographic comparison using UTF-16 code units. This matches how the sort command works on most Unix systems. For locale-aware sorting (where accented letters sort between their base letter and the next one), you would need new Intl.Collator().compare as the comparator.
Worked Examples
The default input has 9 lines: apple, banana, apple, cherry, banana, date, apple, cherry, elderberry. Without any options, the tool outputs 5 unique lines (apple, banana, cherry, date, elderberry) and removes 4 duplicates. The first occurrence of each line is kept in its original position.
Enable the sort option and the output becomes alphabetically ordered: apple, banana, cherry, date, elderberry. Enable case-insensitive mode and “Apple” and “APPLE” would be treated as duplicates of “apple” — the first casing encountered is the one kept.
For a server log with timestamps, leave sorting off to preserve chronological order. The tool removes duplicate warning messages but keeps the first timestamped occurrence, so you can see when each issue first arose.
When to Use This Tool
- Cleaning email lists before importing to a marketing platform (duplicates cause double-sends).
- Removing duplicate URLs from a sitemap or crawl export.
- Deduplicating keyword research exports from Ahrefs, SEMrush, or Google Search Console.
- Cleaning server logs to see only unique error messages.
- Preparing product SKU lists for inventory reconciliation.
- Removing duplicate entries from a contact export (vCard or CSV).
- Building word lists for puzzles, spelling bees, or vocabulary training.
Limitations & Disclaimer
This tool performs exact-match deduplication (or case-insensitive exact match). It does not perform fuzzy matching, so “John Smith” and “John Smith” (double space) are different lines unless trim is enabled. It does not detect near-duplicates like “John Smith” vs “Jon Smith.” For fuzzy deduplication, use a record-linkage library. The sort option uses lexicographic comparison, not locale-aware sorting — accented characters may sort after all ASCII characters. See our disclaimer for full terms. Related: the Find and Replace tool and the Text Diff tool.
Frequently Asked Questions
Does the tool preserve the order of the original lines?
Yes. By default, the first occurrence of each line is kept in its original position. Subsequent duplicates are removed. If you want alphabetical output instead, check the "Sort result alphabetically" option.
How does case-insensitive mode work?
When enabled, lines are compared after converting both to lowercase. So "Apple," "APPLE," and "apple" are treated as duplicates. The first casing encountered is the one kept in the output - the tool does not normalize to a canonical case.
What is the difference between trim and remove empty lines?
Trim strips leading and trailing whitespace from each line before comparing (so "apple " and "apple" are treated as the same line). Remove empty lines filters out lines that contain only whitespace or are completely empty. You can enable either or both.
Is there a limit on how many lines I can process?
There is no hard limit, but the tool processes data in your browser's memory. Tens of thousands of lines work fine; hundreds of thousands may cause a brief freeze depending on your device. For million-line files, use a desktop tool like "sort -u" on Unix.
Does the tool handle Windows line endings?
Yes. The browser normalizes carriage-return-plus-newline to a single newline when reading from a textarea, so Windows-format files work correctly. The output uses newline line endings; if you need Windows endings, open the result in a Windows text editor and re-save.
Is my data uploaded anywhere?
No. All processing happens in your browser. Your data - including email addresses, customer lists, or proprietary data - never leaves your device.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team