I don't understand the extent, but I do understand duplicity. I've always had two sets of ratchets and sockets, for instance, even being an occasional mechanic. Lack of public transit, or bikeable distances to tool shops was a problem, especially when the car I was rebuilding was my own. Things fail along the way, so I have at least doubles of everything. Also, without at least a second set, lots of things were very difficult, such as suspension bolts that needed another ratchet and socket on the other side due to being countersunk on both sides, for instance.
Now, I'm in Germany, where Hazet, Gedore, Stahlwille, and Wera toolsets (among others) are way cheaper than in the US, without even a comparison. They're still pricey, but pro-quality. In that sense, while I'm planning on not staying here, it would be a nice souvenir of the experience to bring back some nice tool sets, even with duplicity to my Armstrongs, CM Pros, and some assorted other truck brands thrown in there at random.
Also, different ratchet types help different situations, as do different types of wrenches, but those really aren't redundant.
Next, good deals come up and they are certainly something that helps from time to time.
So I'll be going back with my original set of tools, but I'll also have all three sizes of Zyklop in there, so my sockets will be duplicated, although with higher quality ones. I'll also be going back with all three usual sizes of Hazet sets, again, duplicating sockets, extenders, etc. Along with that will go some Hazet wrenches, bits, and other stuff.
If a good deal comes up on Facom (dirt cheap anyhow), I'll probably pick up a set as my everyday to go with the Armstrongs and CM stuff.
So the degree of duplicity is what I don't understand, but I think duplicity to some extent is perfectly fine because it can bail you out of something.
And of course, pros need a few of everything because downtime can cost money if they have to wait for a truck to come around or a warranty return by mail. Nothing shameful about techs having CM. It keeps them working.
And with ratchets, sometimes there are different applications where minute differences come into play, such as strength, head dimensions, fine tooth vs course tooth, angle, etc.