def METHOD_NAME(self, obj):
    """Set properties only if they don't exist."""
    if "Group" not in obj.PropertiesList:
        _tip = QT_TRANSLATE_NOOP("App::Property",
                                 "The objects that are part of this layer")
        obj.addProperty("App::PropertyLinkList",
                        "Group",
                        "Layer",
                        _tip)