I wrote:
> Here's a patch to inhibit the same package (in the sense of eq?) from
> being traversed more than once by fold-packages.  One example where this
> helps is the guile-2.0 package in (gnu packages guile), which is
> exported in two different variables: guile-2.0 and guile-2.0/fixed.
>
> Note that even after applying this patch, there are still cases where
> "guix-package -A ." produces the same output twice, but that's a
> different problem.  [...]

Sorry, the patch I posted did not apply cleanly, because it was layered
on top of a preliminary version of my upgrade patch.  Here's a fixed
version.

     Mark