Remove Duplicate Lines
Remove repeated lines while keeping the first occurrence of each exact line. Unlike a sort-and-uniq workflow, this operation does not reorder the input.
What this tool actually changes
The tool reads the input as lines, keeps a set of lines already seen, and emits a line only the first time it appears. Equality is exact: capitalization, spaces, and punctuation are part of the comparison.
Edge cases to know
A line with leading spaces is different from the same text without them. An empty line is also a line, so use Remove Empty Lines first if blank lines should disappear. CRLF and LF input are accepted and the output uses LF.
Example
Input: alpha beta alpha gamma beta Output: alpha beta gamma
Good use cases
Pasted identifiers, configuration entries, import lists, log fragments, and other line-oriented data where original order carries meaning.
Decide whether order and formatting are meaningful
Use this when the first-seen order is useful, such as a list of discovered hosts or a sequence of preferred configuration entries. It keeps only the first exact match: alpha, Alpha, and alpha remain distinct. If formatting differences should not matter, trim or otherwise normalize a copy first and inspect the changed values.
When not to use it
Do not deduplicate chronological logs just because messages repeat; repetition may be evidence. Avoid treating rows of CSV or structured data as plain lines when duplicates should be determined by selected fields rather than the full serialized row.
Complexity and practical limit
The implementation makes one pass and stores each distinct line in a set, so work and memory grow with the input size. Very large files are better handled locally with a script or command-line tool, where you can preserve a source copy and choose exact comparison rules explicitly.
Frequently asked questions
Does this ignore capitalization?
No. Comparison is exact, so Alpha and alpha are different lines.
Does it preserve order?
Yes. The first occurrence is kept and later identical lines are removed.
What about whitespace?
Whitespace is significant. Trim the input first if leading or trailing whitespace should not distinguish lines.
Does the output keep the original line endings?
No. CRLF and LF are accepted, then the output lines are joined with LF. Use a file-aware conversion workflow if preserving the original line-ending convention is required.
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