How to Remove Duplicate Lines from Text & CSV Files While Preserving Order

Whether you are preparing an email broadcast, cleaning server log entries, or organizing keywords, duplicate lines clutter datasets and inflate processing costs.

Order-Preserving Deduplication vs. Sorting

Traditional Unix command-line utilities like sort | uniq remove duplicate lines, but they force the output into alphabetical order, destroying your original chronological sequence.

Modern in-browser deduplication uses a JavaScript Set data structure:

function deduplicatePreserveOrder(textLines, caseSensitive = true) {
  const seen = new Set();
  return textLines.filter(line => {
    const key = caseSensitive ? line : line.toLowerCase();
    if (seen.has(key)) return false;
    seen.add(key);
    return true;
  });
}

• Time Complexity: $O(N)$ linear time.
• Order Guarantee: The first occurrence of each unique line is maintained in its original spot.

Use DevKitLab's Remove Duplicate Lines Tool to clean large datasets privately in your browser.