Duplicate Line Remover: decide what makes two lines equal

Line deduplication can be exact or it can ignore case, leading spaces, or trailing spaces. Use a short sample containing repeated lines, a blank line, and two lines that differ only by capitalization. Check whether the first or last occurrence is kept and whether original order remains stable.

Line endings matter when byte-for-byte output is required. Report the copyable input, normalization options, expected kept lines, and actual output. A tool should not silently trim significant whitespace.