In my story I've got a shop called "Don's Bait Shop" or "Don's" for short. If I then want to describe something belonging to "Don's" do I have to add a further possessive? For example, if I were to use, say, Wal-Mart: "Wal-Mart's floors were covered in filth." I added the possessive. If I then replace "Wal-Mart" with "Don's" I get: "Don's's floors were covered in filth." But the extra possessive just seems silly. I suppose I can choose how I want to do it, but what is the official ruling?
According to the Oxford Dictionary of American Usage and Style, it would be Don's'. But they recommend rephrasing the sentence to avoid this because it looks clunky, and I agree with them.
If Don is a character in the story, say, the owner of the bait shop, it could be phrased as 'Don's floors were covered in filth', if you were talking about Don's floor, instead of the shop's floor.
If the readers are familiar with the name, Don's, and it can't be confused with the character, Don, it might be OK. But I'd think, 'the bait shop's floors' would be more clear.
