bug#44737: Packages grafted twice during system install

[Ludovic Courtès]

Hello!

While installing 1.2.0rc2 (I selected GNOME), I noticed that different
derivations of the same packages are grafted (screenshot below).

I don’t think it should block 1.2.0 but we should definitely find out
why this happens as this increases installation time and possible disk
usage.

Looking at the first two ungrafted .drvs for libcanberra shown in the
screenshot, both depend on two gdk-pixbuf+svg derivations (!), and one
of them differs.  It appears that one of them depends on
libx11-1.6.9.drv while the other depends on libx11-1.6.A.drv.

What I observe here seems different:

--8<---------------cut here---------------start------------->8---
$ guix gc -R $(guix system build ~/src/guix/gnu/system/examples/desktop.tmpl 
-d)|grep libcanberra-0.30.drv
/gnu/store/qbgfsgmlrnq8gklv8v924j1brqvxk6fl-libcanberra-0.30.drv
/gnu/store/l7413kkgxs04jflafah895034a5rd1kx-libcanberra-0.30.drv
/gnu/store/cpgv7xkv2fzr4vj2wqlcbjrwslshq78y-libcanberra-0.30.drv
$ guix describe
Generacio 166   Nov 16 2020 11:22:04    (nuna)
  guix 74eeb11
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 74eeb11daee906cb012f10b6bb3afd254f9ea5c2
--8<---------------cut here---------------end--------------->8---

There are two grafting derivations and one non-grafting.  The two
grafting derivations differ in their GTK+ input, presumably because of
multiple outputs.

Ludo’.