Well as i know and recently I got married with duldung. You can marry with valid duldung when you other required documents for marriage ( valid passport,birth certificate,unmarried certificate and no objection certificate for marriage) for every Standesamt papers are different so is better ask to this Standesamt where you live pr your fiance live that which papers you need for marriage,it also different from country to country that which papers Standesamt need ans how long they need for verification I hope this answer will help you if not you can ask to me because I was recently in same marriage procedure.