Have just one InfoWindow open in Google Maps API v3

One smart easy way to do this with jQuery is the following :

            google.maps.event.addListener(marker, 'click', function (e) {
                jQuery(".gm-ui-hover-effect").click();
                marker.info.open(map, this);
            });

It will click on all the closing buttons amongst your tooltips.

My approach allows you to toggle the infoWindow as well.

Global space

var infoWindow = new google.maps.InfoWindow();
infoWindow.setContent(contentString);

var lastInfoWindow;

Local space

marker.addListener("click", (e) => {
  if (lastInfoWindow === e.domEvent.srcElement) {
    infoWindow.close();
    lastInfoWindow = null;
  } else {
    infoWindow.open({
      anchor: marker,
      map,
      shouldFocus: false,
    });
    lastInfoWindow = e.domEvent.srcElement;
  }
});