To find duplicates, I prefer to create two temporary helper columns.
If my base info is in columns A and B with data starting on Row 2 downward,
where with header A1 reads "FName" and B1 reads "LName":
A B C
D
1 FName LName CName If Duplicate, Say So!
2 Joe Jones =A2&B2
=IF(COUNTIF(C$2:C2,C2)>1,"Duplicate","")
3 Paul Smith (Copy from C1 and D1)
4
Column C is a concatenation of A and B forming a unique value.
Column D tests that value for uniqueness.
You can then do an auto-filter on Column D selecting "Duplicate".
HTH
JMay