Comment 1 for bug 750787

something like

this.openwindow = function(i) {
     if (infowindows[i] != null) {
             for ( var n in this.nodes )
             {
               marker[n].infowindow.close();
              }
               infowindows[i].open(this.map, markers[i]);
      }
}

code hasn't been tested though...