int double_click = 16; //allow approx 1/2 second for double click

function double_left_click(){
  ...
}

function single_left_click(){
  ...
}

function mouse_left_click(){
  while(mouse_left){ wait(1); }
  int i;
  for(i=0; i < double_click; i++){ //will check number of frames possible to be able to allow double click
    if(mouse_left){ //if mouse clicked again perform double_left_click action
      double_left_click();
      return;
    }
  }
  single_left_click(); //if mouse not clicked within double_click time, perform single_left_click action
}

void main(){
  on_mouse_left = mouse_left_click;
  ...
}