CSV Deduplicator

Duplicate addresses in a mailing list mean people get the same email twice, which annoys subscribers and can hurt your sender reputation. Merging lists from several sources — a shop export, a newsletter export and a spreadsheet of event sign-ups — almost always creates duplicates. Run the merged file through the deduplicator with the email column ticked and Ignore upper / lower case on, keep the last copy if your newest data is at the bottom, and save the removed rows to review them.

Comparing whole rows removes only exact repeats, which is safe but misses near-duplicates. Comparing key columns finds records that describe the same thing even when other details differ.

When rows share the same key, only one is kept. Keep first keeps the copy that appears earliest in the file — usually the original entry. Keep last keeps the latest copy, which is often the most recently updated record when the file is sorted by date or appended over time. Either way, the kept rows stay in the order of the original file, so nothing is reshuffled. The removed rows can be downloaded separately to check them or merge details by hand.

People type the same email address in different ways — with capitals, with a space copied at the end. With Ignore upper / lower case and Ignore spaces around values on (the default), those count as the same value. Only the comparison ignores them: the rows you keep are saved exactly as they were. Switch the options off for codes where case matters, such as some product codes or case-sensitive IDs. For other hidden differences, run the CSV Cleaner first.

Customer and subscriber lists are personal data. This tool reads your CSV in your browser and never uploads it; the duplicates are found on your device and nothing is stored. The separator is detected automatically and kept in the saved files, quoted values and line breaks in cells are handled correctly, and files up to about five million cells can be processed.