I've seen a comment from about deduplication but I cannot find it now in GitHub's UI...
If setMarkerCluster() is called several times then this behavior will be added several times. Each instance will contribute to the header. Wicket will render only once the contribution though.
You can use something like:
````java
if (getBehaviors(GMapMarkerClustererHeaderContributor.class).size() == 0) {
  add(new GMapMarkerClustererHeaderContributor());
}
````

BTW is there a typo in GMapMarkerCluster*er*HeaderContributor ?