Well, how are we supposed to help here?
It would really help us to help you if you could tell us the expression you are using, unless you want us to start wild guessing (some other information about your data model etc might also help).
Maybe you do, so I assume you are using an advanced aggregation in your expression (i.e. the aggr() function).
As I said, just guessing.
Try adding a NODISTINCT qualifier to the aggr() function, like sum(aggr( NODISTINCT sum(FIELD), DIMFIELD1, DIMFIELD2))