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.