You can do this with a simple bit of spreadsheet manipulation. Just sort by whatever column you're checking for duplicates. Then, next to that column, insert a new blank one. Suppose that column you've sorted by is D, and the blank column is C. Suppose the data starts at the fifth row, or D5 (just to pick a number). Then in the blank column, in position C6, just type in the formula, =D5=D6. This will evaluate to TRUE if D6 is a duplicate of D5..
Then, with square C6 selected, click the bottom right corner of the bold rectangle (surrounding C6) and drag the selection down the entire column. The formula will copy itself, instead with =D6=D7, then =D7=D8, and so on.... You will see 'TRUE' wherever there is a duplicate, and you could find them quickly via Edit > Find....
If your definition of "duplicate" involves more than one equal column, you can make a trivial modification to your formula, something like =D6=D7 and E6=E7, or whatever syntax Excel uses.
You could also save the spreadsheet as a CSV file (comma separated values), write a CSV parser in Perl (or C++, but if you're familiar with Perl or Python or Ruby or ...., life is much easier), and search for duplicates in a small program. But it won't be as easy. I just had to do this exact thing at work (parse CSV files, find duplicates, but a bit trickier), and if you're willing to wait until Monday, I could send you a simple CSV-parsing Perl module (which is not guaranteed to work on all inputs, and would only be useful if you... knew Perl).
However, I recommend using the features of Excel.