So for this task First Sort your data in order from A to Z or Z to A then you can just use one simple formula as stated below:
=IF(A2=A3, "Duplicate", "Not Duplicate")
The above formula states that if column A2 data ( A is column and 2 is row number) is similar to A3 (A is Column and 3 is Row number) then it will print Duplicate else will print Not Duplicate.
Lets consider an example, Column A consists Email address in which some are duplicate, so in Column 2, I used the above stated formula which in results displayed me the 2 duplicates cells one is Row 2 and Row 6.
One you got the duplicate data just put filter on your sheet and make visible only the duplicate data and delete all the unnecessary data.