Duplicate Line Remover

Lists accumulate duplicates constantly: an email list built up over several exports, a set of URLs scraped from different pages, a list of tags or keywords typed by different people at different times, or a CSV column pasted into a text box for a quick check. Manually scanning a long list for repeats is slow and error-prone, especially when two "duplicate" entries differ only by capitalization or a stray trailing space that is easy to miss visually. This tool treats each line of the input as one list item and removes repeats according to rules you control, rather than a single fixed definition of "duplicate." The case-sensitive comparison toggle controls whether "Apple" and "apple" are treated as the same entry or two different ones. It defaults to on (case-sensitive), meaning "Apple" and "apple" are kept as separate lines unless you turn it off — this matches how most programming contexts and exact-match use cases (like comparing exact string values from a database) expect duplicates to be judged. Turning case-sensitivity off is useful for lists where capitalization is inconsistent but semantically meaningless, like a list of email addresses or tags where "Marketing" and "marketing" really do mean the same thing. The trim-before-comparing toggle controls whether leading and trailing whitespace on a line is ignored when checking for duplicates. It defaults to on, because trailing whitespace is one of the most common invisible causes of a "duplicate" not being caught by a naive exact-match comparison — a line copied from a spreadsheet cell often carries a trailing space or tab that is visually indistinguishable from a clean line but would otherwise prevent two identical-looking entries from being recognised as duplicates. Importantly, trimming here only affects the *comparison* — the lines that remain in the output keep their original content exactly as typed, including any leading/trailing whitespace they had. This is a deliberate choice: the tool's job is to decide which lines are duplicates of each other, not to silently rewrite the content of the lines you keep. The keep first / keep last choice matters whenever the different copies of a "duplicate" line are not byte-for-byte identical after the whitespace/case rules are applied for comparison but there was more than one occurrence in the raw input — practically, it decides the output's line order and which literal occurrence (with whichever original spacing/casing it happened to have) survives into the result. Keep First scans top to bottom and keeps the earliest occurrence of every distinct line, dropping every later repeat; the surviving lines therefore appear in their original top-to-bottom order. Keep Last does the mirror image: it scans and keeps the latest occurrence of every distinct line, dropping every earlier repeat — the result still comes out in ascending original order, but wherever a line repeated, the version and position that survives is the last one in the original list rather than the first. The stat line beneath the controls reports exactly how many lines were removed as duplicates and how many unique lines remain, giving an at-a-glance summary without needing to manually count the before/after line totals yourself.