怎样禁止程序重复运行?

handle:=openmutex(mutex_all_access,false,'ddddd');
        if handle=0 then
        begin
           handle:=createmutex(nil,false,'ddddd');
           if handle=0 then
           begin
              showmess(3,'创建程序同步出错!!!','注意');
              exit;
           end;
        end else
        begin
           // showmess(3,'该程序已运行!!!','注意');
            exit;
        end;