Fixes <https://bugs.gnu.org/32966>.
Reported by Clément Lassieur <clement@lassieur.org>.
* guix/gexp.scm (gexp-attribute): Add 'equal?' optional parameter; pass
it to 'delete-duplicates'.
(gexp-modules)[module=?]: New procedure.
Pass it to 'gexp-attribute'.
* tests/gexp.scm ("gexp-modules deletes duplicates"): New test.