Problem with master calendar

[#1]

Good day,

I use a master calendar to link sales and invoice, but my "sum(...)" gives me too many (the number of days in month times more).

Why does my values are counted more than 1 time ?

How can I avoid that without creating a separate table for years and months ?

=sum(if(Sales_Year = Year and Sales_Month = Month, Sales_Value))

I can't use "distinct" because sometimes, my sales have the same amount...