Finding Repeated Items in Excel 2003

[Guest]

I'm working on a Spreadsheet where Column A and Column B are filled with
E-mail addresses. I created the following formula to find out what E-mail
address in Column A repeats in Column B and vice versa, but it does not work:
=ISNA(MATCH(B2,$A$2:$A4661,FALSE)) All i get is True for everything
including the blank cells.
I try to do the reverse and see what e-mail address in each column does not
repeat, but it did not work.

Any Help?

Thanks,

 

[Guest]

Ok, so now i tried this formula:
=SUMPRODUCT(($A$2:$A$662=A2)--($B$2:$B$662=B2)) but when it encounters a
blank cell in column b it counts the items as 7.

What am I doing wrong.