Give a running count (1, 2, 3…) each time a value repeats down a column.
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=COUNTIF($A$2:A2, A2)The expanding range $A$2:A2 grows as you fill down, so COUNTIF returns how many times the value has appeared up to and including this row — effectively numbering each occurrence.
=A2&"-"&COUNTIF($A$2:A2, A2)→Order-2Creates a unique key per duplicate.
A fixed range gives the same total in every row; the expanding range gives each row its position in the sequence.
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