Depends on whether you were taught the old or new rules, and whether you remember them correctly.
The old rules said you only write the 3 same consonants only if they are followed by another consonant. Schiffahrt with 2 f is correct under the old rules, because the f is not followed by another consonant. But it would be Schifffracht (ship cargo) with 3 f because of the last f being followed by a consonant. Or Hellila (light purple) with only 2 l in the middle instead of three (the word is a compound of hell and lila).
The new "simplified" rules from 1996 say you always write the 3 same consonants or vowels. Alternatively you can use a hyphen. Schifffahrt with 3 f is correct under the new rules. Schiff-Fahrt is as well. Schifffracht or Schiff-Fracht. Helllila or Hell-Lila.
While the rules are taught (and following the rules is part of the grade you will get), they are now lawfully binding in German-speaking countries, and indeed there are plenty of people, especially those who went to elementary school before 1996, and even publications that use the old rules still.
I was in school in the middle of it all. I was taught the old rules in elementary school but the new rules technically came into effect while I was in mid/high school. We were given a short introduction to the changes, and were allowed to use whatever rules we wanted. Some teachers would mark you down for spelling mistakes if you mixed new and old, tho.
Wikipedia claims that Donaudampfschiffahrtsgesellschaft is the proper name of a company that only existed until 1991, so the new rules that were introduced in 1996 do not apply. The company never changed its name to the new spelling, so the old spelling should be used to refer to the historical company. The reasoning makes sense to me.
If you're referring to that company specifically, yes, their spelling would not have changed, as it is an Eigenname. The actual name was Erste Donaudampfschiffahrtsgesellschaft (Erste = first). However there were probably a bunch of companies operating steam ships on the danube, which can be reasonably called Donauschifffahrtsgesellschaften ;)
