pay attention, the use of one at every side is not forced and depends on chassis/ alignement of rear axles!!! There are cars , who need 1 ,2 or 4 !!! To put them right or left , depends of the stand of the rear axle! Please advertise your local specialist.
you have to take measurements first of what exactly washer you need , alfa used 4 types 1.5mm-3mm. If you need for example 2mm in one side and you fit one 3mm , yes you will have a problem. In your copy shown before there is also a special washer (S) it is a washer which also must fit after taking measurements as manuals prompt. Alfa also supplied these S washers in difference thickness. I might have some NOS of these S washers and maybe also some of the triangle type to body washers. Need to check