This fixes the inconvenience occuring in `cabal.project` files of
separate `allow-*` statements overwriting each other. Currently,
without this patch,

    allow-newer: base
    allow-newer: directory

is *NOT* equivalent to

    allow-newer: base, directory

but rather to the last occurence of `allow-newer`, i.e.

    allow-newer: directory

This behaviour has been present since cabal 1.24