First double negatives and now double possesives?!

I was watching the news today, and the title of a story they presented was “Legacy of Don Knott’s”.

Now, at first glance, I was positive that it was a grammatical mistake. I mean, why say “Legacy of Don Knott’s” when saying “Legacy of Don Knott” would do the job?

But then I replaced “Don Knott’s” with “his” (the phrase thus becoming “legacy of his”) and the latter phrase seemed to make sense. We say things like “that book of his”, so why not “legacy of his”?

So here comes the question: Are both the phrases “Legacy of Don Knott’s” and “Legacy of Don Knott” correct? Is there such a thing as double possesive? And why, for goodness sakes, can’t we just simply say “Don Knott’s Legacy”?!

And whatever happened to the man, anyway? Why are they all of a sudden presenting a story on his legacy? (Or, shall I say, legacy of his.)