Excel compare two columns and highlight duplicates

Question

I have an excel file with 10,000 rows in column A some values are the same.

Example:

A1 - P7767

A2 - P3443

A3 - P7767

A4 - P8746

A5 - P9435
         


        

Answer 1

A simple formula to use is

=COUNTIF($B:$B,A1)

Formula specified is for cell A1. Simply copy and paste special - format to the whole of column A