Need help with a formula, please, I have a MS Excel spreadsheet with

A1=LAST NAME
B2=DATE OF BIRTH

and I need a formula to show me all instances where the last name AND the
date of birth repeat. I found EXACT formulas, AND formulas, OR formulas, and
even used Conditional Formating with a COUNTIF formula. But non of them are
able to perform a cross comparision or at least I don't know how to enter it
properly.