I am not able to place markers on the places which is already marked by google with other symbols .

Question

The info window and marker is not shown for the places which is already marked by google. for example: if you run the code and click anywhere , a marker and a info window will a

Answer 1

When you click on such a place(POI) there will no click be triggered on the map.

When you don't want to hide the POI's, you must trigger the click on your own.

Possible approach(see: How to get a click event when a user clicks a (business) place on the map )

//keep a reference to the original setPosition-function
var fx = google.maps.InfoWindow.prototype.setPosition;

//override the built-in setPosition-method
google.maps.InfoWindow.prototype.setPosition = function () {

  //this property isn't documented, but as it seems
  //it's only defined for InfoWindows opened on POI's
  if (this.logAsInternal) {
    google.maps.event.addListenerOnce(this, 'map_changed',function () {
      var map = this.getMap();
      //the infoWindow will be opened, usually after a click on a POI
      if (map) {
        //trigger the click
        google.maps.event.trigger(map, 'click', {latLng: this.getPosition()});
        //hide the default-infoWindow of the POI
        this.setMap(null);
      }
    });
  }
    //call the original setPosition-method
  fx.apply(this, arguments);
};

Demo: http://jsfiddle.net/doktormolle/1yrpm98q/