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This would require you to sort the data. If that is an option, this might be easier for some. I insert a column to the right of the one with potentially duplicate data. (Say potential duplicates are in column A, I insert a blank column at B.)

First row is usually headings that will not match the data itself. (I'll change the heading for A if I have to.) Let's assume for this that the data starts in row 2 with headings in row 1.

I put a formula in cell B2 as follows: =if((a2=a1),"Dup","-")

I get 'Dup' if there is a duplicate, '-' otherwise.

I hope this helps.