Fix for an obscure `cascading union` use case in RectangularMesh

Addresses https://bugs.launchpad.net/openquake/+bug/1133447.

This is handling for a special case has only consistently cropped up in our CI environment: http://ci.openquake.org/job/oq-engine/199/testReport/

Basically, this part of the code throws an exception when trying to create a cascading union of many polygons (resulting in a single polygon). This seems to happen only with VERY specific geometry AND when one of those very specific geometries is duplicated in the input list to shapely.ops.cascading_union.

Oh, it gets worse. The only machine on which I've been able to reproduce this error is the CI machine. No other machine can do it as far as I know.