prevent crash on deduplicating empty set
prevent crash on deduplicating empty set

amcat/amcat,amcat/amcat,tschmorleiz/amcat,amcat/amcat,tschmorleiz/amcat,amcat/amcat,amcat/amcat,tschmorleiz/amcat,tschmorleiz/amcat,amcat/amcat,tschmorleiz/amcat