I need to summarize sales by month for different book titles. Following this post Invoices report-grouping by year and month...how? I created a field called calcMonth with this formula: Date ( Month ( salesDate ) ; 1 ; Year (salesDate ) ). I placed it in a subsummary part using that same field. It displays properly, and there are results grouped by month. But rather than displaying just one instance of each month, there are duplicate months.