Remove Duplicate Lines Remove duplicate lines from text, keeping unique lines only.
Remove Duplicate Lines
Remove duplicate lines from text, keeping unique lines only.
Paste Lines to Deduplicate
Enter text with potential duplicate lines.
Configure Deduplication Options
Choose case-sensitivity and whitespace trimming options.
Review Unique Lines
See deduplicated text with count of removed duplicates.
What Is Remove Duplicate Lines?
A Remove Duplicate Lines is a utility that removes duplicate lines from text data, preserving only the first occurrence of each unique line. Developers and data analysts use it to clean up log files, deduplicate lists, and process text output that may contain repeated entries. One specific problem it solves is removing repeated lines in a list of email addresses or URLs, which can occur when scraping data from websites or processing user input.
The tool stands out due to its configurable options, such as case-sensitive or case-insensitive comparison, allowing users to choose how duplicates are identified. It also offers optional whitespace trimming before comparison, which is useful for removing duplicate lines that differ only in leading or trailing spaces. This feature is implemented using the `trimWhitespace` state variable and the `toLowerCase()` method for case-insensitive comparison.
It provides a simple way to remove duplicate lines online, making it easy to clean up text data without having to write custom code. By using this tool, users can delete duplicate lines from large datasets quickly and efficiently, resulting in deduplicated text that contains only unique lines. The output is formatted as plain text, with each line separated by a newline character (`\n`), making it easy to copy and paste into other applications or further process the data.
Why Use Remove Duplicate Lines?
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Remove exact and fuzzy duplicate lines
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Options for case-sensitivity and whitespace handling
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Shows count of removed duplicates
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Preserves original line order
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Processes large text files in a single pass
Common Use Cases
Data Cleaning
Remove duplicate entries from lists, exports, or log files.
Email Lists
Deduplicate email address lists before sending campaigns.
Log Analysis
Filter log files down to unique events by stripping repeated entries.
Code Cleanup
Clean up source files by eliminating duplicate imports, includes, or declarations.
Technical Guide
Deduplication uses a Set data structure to track seen lines. For each input line, a comparison key is generated (optionally trimming whitespace and lowercasing), and if the key hasn't been seen, the original line is added to the output and the key is added to the Set. This preserves the first occurrence of each unique line while removing subsequent duplicates. The algorithm is O(n) in time complexity (one pass through the input) and O(n) in space (for the Set). The count of removed duplicates is calculated as the difference between input and output line counts.
Tips & Best Practices
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1Enable whitespace trimming to catch duplicates that differ only in spacing
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2Case-insensitive mode catches "Hello" and "hello" as duplicates
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3The tool keeps the first occurrence and removes subsequent duplicates
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4Combine with Sort Lines A→Z for sorted, deduplicated output
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5Check the removal count to see how many duplicates were found
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📝 Text ToolsFrequently Asked Questions
Q What occurrence of a duplicate line is kept?
Q Is it case-sensitive?
Q Does whitespace matter?
Q Is the original order preserved?
Q Can I see what was removed?
About This Tool
Remove Duplicate Lines is a free online tool by FreeToolkit.ai. All processing happens directly in your browser — your data never leaves your device. No registration or installation required.