Finding rows with duplicates in two columns

Can someone please help me with a formula or vb code to find all of the rows that match another row on two columns? I need to be able to delete all duplicate rows, but they're only duplicates if two columns match. For example

ROW   A    B
 1    1    a
 2    2    a
 3    3    b
 4    4    b
 5    1    a

Only rows 1 and 5 would match.