I used to use a conditional formatting formula when comparing two columns to highlight the duplicate cells in the second column. I know the formula begins =COUNTIF ($A:$A but that's all I can remember.
For context: when comparing ID numbers in Column A to ID numbers in Column B I want the IDs that appear in both Column A and Column B to be highlighted in Column B only.
The closest I can get is: =COUNTIF($B:$B, A1) but it's highlighting the ID #s in both columns.