Adding multiple markers to gmap in winforms

I am trying to plot custom markers on lat-lon points read from a csv file

But I am not able to get my code to work: it either adds just the first marker or the last

Could you please have a look and tell me where i am getting it wrong?

   markers = new GMapOverlay("markers");
                 while ((line = sr.ReadLine()) != null)
                {
                    string[] parts = line.Split(',');
                    var p = parts;
                    Double.TryParse(parts[3],out lat);
                    Double.TryParse(parts[4], out lon);
                    GMapMarker marker = new GMarkerGoogle(new PointLatLng(lat, lon), new Bitmap("..\\..\\Resources\\arp.jpg"));
                    
                    marker.ToolTipText = new PointLatLng(lat,lon).ToString();
                     
                    marker.IsVisible = true;

                    markers.Markers.Add(marker);
                    navDataMap.Overlays.Add(markers);

 

                }

//adds just first marker

                markers = new GMapOverlay("markers");
                 while ((line = sr.ReadLine()) != null)
                {
                    string[] parts = line.Split(',');
                    var p = parts;
                    Double.TryParse(parts[3],out lat);
                    Double.TryParse(parts[4], out lon);
                    GMapMarker marker = new GMarkerGoogle(new PointLatLng(lat, lon), new Bitmap("..\\..\\Resources\\arp.jpg"));
                    
                    marker.ToolTipText = new PointLatLng(lat,lon).ToString();
                     
                    marker.IsVisible = true;

                    markers.Markers.Add(marker);
                    //navDataMap.Overlays.Add(markers);

 

                }
                navDataMap.Overlays.Add(markers);

//adds just last marker

Thank you