Comparing data in multiple excel worksheets

Hello,

For work, I was given an excel document with a different worksheet for each month in 2012. Clients can come to our food pantry one time in each calendar month. On the worksheet for each month, column A has the client's last name, B has the first name, and the other columns have information about things like gender and zip code and birthdate. We are trying to find a way to compare the data in the different worksheets, with the ultimate goal of figuring out how many total, unduplicated clients we have seen throughout the year. (If the same person came in July and then October, we only count it as one person.) Does that make sense?? And does anyone know any efficient way to do this???

Thank you very much!