Question: what is the formula in excel to find duplicates?
Answer: Input the above formula in B2, then select B2 and drag the fill handle to copy the formula down to other cells: ... 
=IF(COUNTIF($A$2:$A$8, $A2)>1, "Duplicate", "Unique") ... 
The formula will return "Duplicates" for duplicate records, and a blank cell for unique records: