Report multiplies potentials

Report multiplies potentials

Hello! When I created one summary report grouped by 2 different parameters (owner of potential & stage of potential) with only 1 criteria in a standard filter (no advanced filter), I get a report multiplying certain potentials, but only some of them, although there is definitely only one created under that name.  
For example: 
owner Stela: potentials: - Marco
                                    - Jeany
                                    - David
                                    - Maria      
                                    - Maria
                                    - Maria
... !?
We use more complicated potential names, that's why I assure that each name is unique. 

Any idea? 
Thank you and kind regards
Stela