Conditional formatting to highlight duplicates across two columns

Joshua65 Start by selecting the entire data range of both columns, from A1 to B6 in this example. On the Ribbon, go to Home > Conditional Formatting > New Rule > Use a formula to determine which cells to format and use the following formula:

=COUNTIFS($A$1:$A$6, $A1, $B$1:$B$6, $B1)>1

Take note of the absolute cell references for criteria_range1 and criteria_range2, while both criteria cells are using a mixed reference of absolute columns and relative rows.

Then choose your desired cell formatting and click OK. The results should look something like this:

Conditional Formatting Duplicates in Multiple Columns