List Deduplicate by Key Column
Remove duplicate rows from a delimited list based on just one column — an email, an ID, a reference number — keeping the first row seen for each.
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About this tool
Different from a plain duplicate-line remover: two rows can differ in every other column and still be treated as duplicates here, as long as the chosen key column matches. For tab-separated data pasted from a spreadsheet, type \t or tab as the delimiter.