I am sure whatever reasons there are, they will not be practical for sure... Exception being what was brought up earlier , i.e. two people who have the same system wanting to use the same lens at the same time, e.g. Canon users who both use 50mm f/1.4 to shoot cats.
I normally buy and use old discontinued lenses so if a lens is rare and in great condition, I might just get another copy for use as normal workhorse and the nice one I keep as collection. I also do cherry-picking for certain lenses I particularly like. I may just get a few copies, select the best and maybe the 2nd best and then sell off the rest. So before I sell off the excess I could have a few copies of the same lens
This basically only applies to old lenses with collection value. New lenses I don't really bother...
I do have identical lenses from the same mount for several reasons, First is we shoot with professional big aperture lenses, I have cheap versions to shoot for my own projects. Once I broke the back element of my 85 f1.2 and that thought me to get a backup for my important focal length.
50mm f1.8 for leisure and collection, its a mark1, f1.2 for work, 45mm for creative purposes
85mm f1.8 for small projects, f1.2 for weddings and portraits, 90mm 2.8 for creative purposes