Conditional sum in contact reports based on dates

Hi. I am using a report based on contact and on another module calles payments.
For each contact, I have in the payment module the purchases and amounts of purchase made.
1- I need to make a single report where I can sum for each contact the SUM of his purchases from the module payment BETWEEN 2 specific dates (conditional sum)
2- When suming the payments for each customer, i get a duplicate row for contacts for each payment, when payment are supposed to be grouped (sum)

How to avoid this?

See this post

Uncheck the Group Format checkbox from the Fields Block while Generating the Report as shown in the below screenshot: