Remove Duplicate Lines — Free Online Dedupe

When you need to dedupe

Duplicate rows or lines sneak into data constantly: an email-marketing CSV merged from multiple lists, a URL audit that joined sitemap and crawl data, a customer export run twice, a list of IDs pulled from multiple queries, a backup script that scanned the same folder twice. Removing duplicates is the first step in any data cleanup workflow.

Options explained

• Case-insensitive — treats example.com and Example.com as the same. Useful for emails, URLs, names.
• Trim whitespace — ignores leading/trailing spaces when comparing. Catches duplicates with stray spaces.
• Sort output — returns the deduplicated list in alphabetical order.
• Keep last occurrence — when there are duplicates, normally the first one in the input is kept. Tick this to keep the last instead — useful when later entries have more recent data.

Common deduplication scenarios

• Email lists for marketing — deduplicating across imported sources prevents sending the same campaign twice to one person.
• URL audits — combining sitemap, GA, and crawl data inevitably produces duplicates.
• Customer ID lists — merging exports from multiple regions or product lines.
• Cleaning up CSV row imports before loading into a database with unique constraints.
• Word lists / dictionaries — building unique-word lists for language tools.
• Domain blocklists — combining multiple security feeds without redundant entries.
• Quiz / survey response cleanup — finding duplicate submissions.

Tips

• Always tick "Trim whitespace" for email lists and URLs — invisible whitespace is the #1 cause of "duplicates" that look identical but aren't.
• Case-insensitive mode is right for most identifiers (emails, domains, country codes) but wrong for case-sensitive data (passwords, API tokens, code).
• For CSV row deduplication based on a specific column, this simple line-based tool only catches exact-row duplicates. Use a spreadsheet or database for column-key dedupe.

FAQs

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