Oh nice, duden itself isn't consistent with it's content. Searching "Schiffahrt" gets me the result that the triple f is the old way of writing it: https://www.duden.de/rechtschreibung/Schiffahrt
But I don't understand your last sentence. If both are correct than both are the right way. It's just like "größer wie xy" is correct for a couple of years. Maybe it doesn't sound nice, but it's correct.
Der Duden hat an vielen stellen einfach aufgegeben ein strenges Regelwerk zu sein. Wenn es wegen irgendeinem Wort Uneinigkeit gibt wird einfach beides erlaubt.