How to Avoid double-counting duplicates within a list in Microsoft Excel 2010

New to Microsoft Excel? Looking for a tip? How about a tip so mind-blowingly useful as to qualify as a magic trick? You're in luck. In this MS Excel tutorial from ExcelIsFun, the 726th installment in their series of digital spreadsheet magic tricks, you'll learn how to use the SIGN, COUNTIF and SUMPRODUCT functions to count 1 for each item in a first list that's also contained in a second list.