from the help file:
The double $$ means that this command will only be executed if a parameter is specified. If you specify only one parameter in the above command it will not be executed. You can also do $$?1 or $?1 which means try to fill this value with parameter one if it exists. If parameter one doesn't exist, ask for it. In the first case the parameter is necessary for the command to be executed, in the second case it isn't.