removeOverlay not work properly with polyline and polygon

[penelop]

Hi,
I've created a map that allow user to click on the map and draw a
polyline or polygon.
I placed a button to help user undo the last click and draw the
polygon again, but removeOverlay doesn't work properly, the old
polygon still remains on the map.
Here is my link to the map

http://travelrise.com/test/prototype.html

Here is my code to redraw the polygon

function PolygonManager(gmap,options) {
        var map = gmap;
        var gPolygon = null; //store reference to the GPolygon
        var defaults = {
            lineColor:"#000",
            lineWeight:2,
            opacity:0.5,
            fillColor:"#0f0"
        };
        var opt = options;

        this.update = function(subject) {
                var settings = $.extend(defaults,opt);
            var points = [];
            points = subject.getClickPoints();// receive the GLatLng
array

            if (gPolygon) {
                map.removeOverlay(gPolygon);
            }
            if (points.length > 2) {
                points.push(new GLatLng(points[0].lat(),points[0].lng
()));
                gPolygon = new GPolygon(
                   points,
                   settings.lineColor,
                   settings.lineWeight,
                   settings.opacity,
                   settings.fillColor,
                   settings.opacity
                );
                map.addOverlay(gPolygon);
            }
        }
    }
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