In the variable handler, we just go through the entire thing and keep
every element once.
If there's a duplicate, we set it again, which calls the handler
again.
This takes a bit of time, to be paid on each startup. On my system,
with 100 already deduplicated elements, that's about 4ms (compared to
~17ms for adding them to $PATH).
It's also semantically more complicated - now this variable
specifically is deduplicated? Do we just want "unique" variables that
can't have duplicates?
However: This entirely removes the pathological case of appending to
$fish_user_paths in config.fish (which should be an FAQ entry!), and the implementation is quite simple.
