Check for duplicate registration.

sc2/src/uqm/planets/generate-base.cpp

@@ -203,6 +203,10 @@ namespace Planets  ::detail::  Generate_m
 	void
 	Generator::add( const std::string &name, std::unique_ptr< Generator > gen )
 	{
+		if( registry().count( name ) )
+		{
+			throw std::runtime_error( "Double registration of `" + name + "`" );
+		}
 		registry()[ name ]= std::move( gen );
 	}
 }