when you post this I just lol. two 2x extenders, an abundance of redundant focal lengths. I mean - a 16-35 and a 17-40? Really?
Why not when you can afford it
He may use the lenses as paperweight just to show how wealthy he is at least it is a lot more sophisticated than some of the billionaires who use solid gold tap in their bathrooms
He also has 3 bodies and some particular lenses may work better on one or the other bodies. Maybe one lenses is calibrated for a particular body and then, it stays with that body. I could also imagine setting up multiple bags/kits for different purposes. For example a macro setup, a landscape setup, a sports setup, studio setup, etc. you can get the idea. If one decided that is the way to go, the total equipment list might have a few duplicates/redundancies.
In short - I'm guessing there is a reason for having what seems redundant to some and it has nothing to do with not realizing that a 16-35 and a 17-40 are close to the same FL range. I'd also say that it doesn't necessarily have anything to do with gold water faucettes or L-series paperweights. Some people can afford it and do things for themselves and don't worry if it impresses other people or not.