Assuming your data starts in A1
=COUNTIF($A$1:$A$2000,A1) in cell B1, and copy down to B2000. The formlua will return the number of occurences of the value in A1 in the range A1:A2000. Then you can search for the value 2 to find the duplicate. Alternatively you can sort your data in ascending order on the results, and all the duplicates will be at the top of the file.
HTH
Richard
In a parallel column paste this formula in the second row of data and then copy down to the last row
=MATCH(A2,$A$1:A1,0)
If there is no match it will return N/A, if there is a match it will return the row number of the match.