A few helpful Excel tips can be found in these Tech Republic articles
QUOTE: Counting duplicate values is a subject that begs definition. What constitutes a duplicate? Within Excel, you can have duplicate values within the same column or you can have duplicate records — a row where every value in the record is repeated. In this article, we’ll focus on duplicate values within the same column. Once you start counting duplicates, you’ll often discover that you need more. For instance, you might need the opposite — how many unique values are in the column. There are several ways to count duplicate values and unique values. You can work with most any dataset or download the .xlsx or .xls demonstration file (although specific instructions for the .xls format aren’t included in this article). To get unique values, you can also quickly do an advanced filter, selecting only unique records and copying to another range (In Excel 2010: Data–> Advanced–> complete Advanced Filter dialog).