How to highlight duplicates between two columns?

Question

Google Sheets do not have predefined conditional formatting for duplicate cells. I need to highlight cells in columns A and B that contain a match / duplicate between the two of them.

Answer 1

Please remove formatting from columns A: B, select ColumnA and Format, Conditional formatting ..., Format cells if ... Custom formula and:

=countif(B:B,A1)

Then select the fill fill and Done.

Select ColumnB and repeat with:

=countif(A:A,B1)