In OpenStreetMap there are sometimes multipolygons where attributes are present on both the polygon and the multipolygon relation, such as the Grampians National Park:
These polygons get loaded by OSM2pgsql separately, so I get duplicate polygons in PostGIS. Both match a query like
select * from planet_osm_polygon where leisure='nature_reserve'.
The problem is that when rendering such polygons using a transparent style (eg, over a terrain basemap) they appear darker than other single polygons.
I can't think of any way to avoid this happening, short of not rendering any multipolygons, or only multipolygons.
Any suggestions? Pre-processing in PostGIS is ok if needed.