RunToolRunRemove Duplicate Lines
Remove duplicate lines from text.
How to Remove Duplicate Lines
Paste your list
Drop any text with one item per line — emails, keywords, URLs, names.
Duplicates are removed locally
Repeated lines are filtered out instantly in your browser, keeping the first occurrence.
Copy the clean list
Get a deduplicated list ready for mailing tools, spreadsheets or scripts.
What counts as a duplicate is a decision
Two lines are identical or they are not — but 'identical' has options. Case sensitivity decides whether 'Ankara' and 'ankara' are the same entry. Trailing whitespace decides whether a line with a stray space at the end matches the one without. Both are invisible in the input and both change the result.
For email addresses, product codes and names, case-insensitive matching is almost always what you want, because the difference is a typing artefact rather than information. For passwords, identifiers and anything case-sensitive by nature, it would silently merge distinct values.
Whitespace is where most missed duplicates hide
A list pasted from a spreadsheet, an email or a web page routinely carries trailing spaces, tabs and non-breaking spaces. Those lines are not duplicates of their clean counterparts by exact comparison, so they survive deduplication and reappear in your output looking identical to lines you thought you removed.
Trimming whitespace before comparing removes the whole category. It is the single setting that most changes how well deduplication works on real data.
Order, and which copy is kept
Keeping the first occurrence preserves the original order and is the safe default — a list processed this way looks like the input with fewer lines. Sorting the output groups related entries and makes the result easier to scan, but destroys any ordering the input carried, such as chronology or priority.
Which copy is kept matters when the lines are not truly identical but merely compare equal under your settings. With case-insensitive matching, keeping the first means keeping whichever capitalisation happened to appear first, which may not be the one you would have chosen.
Deduplication is not merging
Removing identical lines is a different operation from combining records that refer to the same thing. Two rows for the same customer with slightly different spellings are duplicates in the business sense and completely distinct as text.
That kind of matching — fuzzy comparison, normalisation, resolving which record is authoritative — is a data cleaning problem rather than a text operation. Exact deduplication is a useful first pass before it, not a substitute for it.
Checking the result
Compare the line counts before and after. If the number removed is far higher than expected, the comparison is probably too loose — case-insensitive matching on data where case is meaningful, for instance. If it is far lower, whitespace is probably not being trimmed.
Processing happens in your browser, so email lists, customer data and internal records are not uploaded. That is the relevant property here, since deduplication is overwhelmingly performed on exactly that kind of personal data.
Duplicates that are not adjacent
A common assumption is that duplicates sit next to each other, which is true only if the list is sorted. In an unsorted list the same value can appear at line 3 and line 4,000, and any approach that only compares neighbouring lines misses it entirely.
Proper deduplication tracks every value it has seen, so position does not matter and the original order can still be preserved. This is worth knowing because the manual approach people use in spreadsheets — sort, then scan for neighbours — is doing the same job less reliably, and a single sorting mistake hides duplicates rather than revealing them.