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.  In the remaining cases, there are actually two
distinct package objects being printed the same way because the
'location' field is inherited from one package to another.  For example
this happens with the 'gcc-boot0' and 'gcc-4.7' packages in (gnu
packages base).

What do you think?

    Mark