This fixes one of the issues noted in <https://issues.guix.gnu.org/50425#4>.
* guix/import/minetest.scm
(import-dependencies): Call 'delete-duplicates' on the resulting list.
* tests/minetest.scm
("minetest->guix-package, multiple dependencies implemented by one mod"):
New test.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>