Hopefully someone can chime in with some tips, thanks!
It's likely you have the facet category added more than once for one or more docs. Like this;
<field name="manufacturer_facet">american standard</field> <field name="manufacturer_facet">american standard</field>Are you adding the facet values on-the-fly? This happened to me and I solved it by removing the duplicate facet fields.
Regards,Jennifer Seaman