How this works in your browser
Lines are processed in order, with each one checked against a set of those already seen and added only if it is new, which is what makes the first occurrence the one retained. Optional trimming is applied before that comparison rather than after, which is the detail that matters most in practice: without it, two visually identical lines differing by a single trailing space are genuinely different strings and both survive. Case-insensitive mode compares lowercased forms while preserving the original casing in the output. Sorting is applied last, so it changes presentation without affecting which entries were kept.
Who uses Duplicate Line Remover
Cleaning email lists
Remove repeated addresses before importing into a mailing system.
Deduplicating exported data
Strip repeats from a list pulled out of another system.
Tidying tags and keywords
Reduce a list to unique entries and sort it.
Merging two lists
Combine lists and remove the overlap in one pass.
Frequently asked questions
Which occurrence is kept when duplicates are found?
The first occurrence of each unique line is kept; later duplicates are removed.
Can it ignore capitalization when checking for duplicates?
Yes - enable "Case-insensitive dedupe" to treat "Example" and "example" as the same line.
Can it sort the result too?
Yes, enable "Sort alphabetically" to sort the deduplicated list before displaying it.
Why are lines that look identical not being removed?
Almost always invisible whitespace: a trailing space on one, a tab on the other, or different line endings from a file that has crossed between Windows and Unix. Enabling the trim option first normalises them, which resolves the great majority of these cases.
When should I dedupe case-insensitively?
For email addresses and usernames, where the same identifier written with different capitalisation is genuinely the same entry. Keep it case-sensitive for anything where capitalisation carries meaning, such as codes, identifiers or proper nouns you want preserved exactly.
Does sorting change which duplicate is kept?
No. Deduplication runs first, keeping the first occurrence of each line, and sorting is applied to the result afterwards. So the entry retained is always the one that appeared earliest in your original order.
Is my list uploaded anywhere?
No. Everything runs in your browser, which matters given these lists are frequently email addresses or customer data.