Searching for duplicate Entries and Combining Data

Hello,

I have a MS Excel document I'm trying to analyze. It is a sales result for particular products. Some entries may be listed twice- for example sales results for year 2007, and sales for 2008. I need to combine the two into one record- combining monthly sales, sales total, and # of invoices. Attached is a sample of the file. Any help is much appreciated. . . .