Remove duplicate lines

How it works

The text is split on line breaks and each line is visited once from top to bottom. The first time a line appears it is kept; every later occurrence of the same line is dropped, so the order of the original list is preserved. By default two lines match only when they are identical strings. Trimming ignores leading and trailing spaces, ignoring capital letters folds the text to lower case first, and removing blank lines drops empty ones on top of that. Each option changes the comparison key rather than the text you get back, so nothing in the kept lines is rewritten.

Worked example

Five lines — apple, banana, apple, Banana, cherry — keep four with the defaults, because the second apple repeats the first while Banana is a different string. Switch on ignoring capital letters and Banana is dropped too, leaving three lines. Switch on removing blank lines as well and any empty row would go as well.

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