Find and destroy duplicate OSM multipolygon/polygons in TileMill/PostGIS

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In OpenStreetMap there are sometimes multipolygons where attributes are present on both the polygon and the multipolygon relation, such as the Grampians National Park:

http://www.openstreetmap.org/browse/relation/2703380 http://www.openstreetmap.org/browse/way/201068605 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.



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