Facet search shows double entries

Add nodes /files/foo and /files/bar of type nt:unstructured

Add property lang=en to files/foo
Add property lang=nl to files/bar

Add node /navigation/test of type hippo:facetsearch
Add properties:
hippo:docbase=files
hippo:facets=lang
hippo:queryname=test

save changes

expand /navigation/test/en/hippo:resultset
=> foo appears twice

close and expand /navigation/test/en/hippo:resultset
=> foo appears three times

close and expand /navigation/test/en/hippo:resultset
=> foo appears four times

etc...