zonggong -
http://www.hitchcocksmotorcycles.com/pa ... Rear_Wheel_ this shows only one washer, part 11 which goes inside the swinging arm and spacer part 6 on the LHS. What (exact) model bike do you have, year-model and geographic region (they vary)?
If I were to try to fit two washers on the RHS then I doubt I would get the brake plate nut on. I cannot imagine why you have two washers there other than faulty assembly, which is not uncommon. But you can do the nut up which means the washers may be thinner. The brake plate thread is large diameter, the washer would not fit the spindle on the LHS. RE do change specs without notice, they may have gone to two washers
Another area of tolerance is wheel dishing where the rim is pulled left or right by the spokes, very common on push bikes with unequal dishing to accommodate the dérailleur.
Your bike is too clean