*handle-warn-on-redefinition* specifies what action should be taken on defining external symbols in the packages specified in the variable *packages-for-warn-on-definition* . If *handle-warn-on-redefinition* is set to :warn then you are warned. If it is set to :quiet or nil , the definition is done quietly. If, however, it is set to :error , then LispWorks signals an error.