The self-weight multiplier controls whether or not structural self-weight is included within a given load pattern. Typically, a self-weight multiplier of 1.0 is only assigned to the load pattern which represents the self-weight of the structure with a multiplier of zero is assigned to all other load patterns. The load case which includes the self-weight load pattern is included within each load combination to account for the self-weight of the structure.
To ensure that self-weight is accounted for only once, check the base reactions listed in the output tables of any load case or combination.