Flag rows that are duplicates in two columns

Question

My question is similar to this one, but adds a little more complexity.

How can I flag duplicate entries in a Google Sheets column unless the duplicates have different values in another column? For instance, consider the table

col A    col B
--------------
abc      123
def      123
ghi      456
ghi      456

How can I flag the last two rows which are the same in both col A and col B but not the first two which differ in col A?

Answer 1

• this custom formula is able to flag 1st (cell) occurrence of a duplicate pair:

=COUNTIFS($A1:$A, "="&A1, 
          $B1:$B, "="&B1)<2

Answer 2

• this custom formula is able to flag and protect all duplicate pairs:

=IF(LEN(A1:A), NOT(COUNTIF(
 ARRAYFORMULA(TRIM(TRANSPOSE(QUERY(TRANSPOSE($A$1:$B), , ROWS($A$1:$A))))),
 ARRAYFORMULA(TRIM(TRANSPOSE(QUERY(TRANSPOSE($A1:$B), , ROWS($A1:$A))))))>1), )