Added two tests to make sure that id's get remapped if an id gets mapped for the second time.

e.g. mapping.map( "hh1", "pc1" ); mapping.map( "hh2", "pc1" ); should result in a mapping between hh2 and pc1 and hh1 should not be in the mapping anymore.

vanRijn corrected this behaviour. This tests make sure that it won't happen anymore in the future.