RenderController : Throw exception on duplicate child names
RenderController : Throw exception on duplicate child names

This situation should not be possible to arrive at if the Gaffer nodes are functioning correctly, but previously this would cause a difficult to debug segfault.

GafferHQ/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,boberfly/gaffer,hradec/gaffer,boberfly/gaffer,GafferHQ/gaffer,boberfly/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,hradec/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,hradec/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,boberfly/gaffer,ImageEngine/gaffer,appleseedhq/gaffer