I have a filter on Print Composer that selects "id_mov" and shows figures one by one into the map. This "id_mov" is not unique so different id can have the same "id_mov". Qgis select distinct by default and for the same "id_mov" I can't find the way to show them all together in the same map composition. It could be like saying in SQL "no select distinct"
I studied your questions, because sometimes I need it..
My suggestion is depends of your format:
Case A: Shapefile Create a new shapefile using "Dissolve" Tool
Case B: PostGIS: Create a View using "Group by"