public Field addField(Field field) {
    	verifyInheritance(field);
        for (Iterator<Field> i = docType.fieldIteratorThisTypeOnly(); i.hasNext(); ) {
            if (field.getName().equalsIgnoreCase((i.next()).getName())) {
                throw new IllegalArgumentException("Duplicate (case insensitively) " + field + " in " + this);
            }
        }
        docType.addField(field);
        return field;
    }
Override getField, as it may need to ask inherited types that isn't registered in document type.
config-model/src/main/java/com/yahoo/schema/document/SDDocumentType.java