Two copies of same library in store after cleaning generations and gc

[Mike Jones]

Hi Guix!

I've been running Guix System for a few weeks now, and I've noticed that
I have two copies of some libraries in my store (e.g. mesa, gtk) even
after updating all packages, deleting old profile generations, and
running gc.

This isn't a problem, as I have plenty of disk space, but I'm just
curious to understand why this happens.

For example, I have both

  /gnu/store/lcqz4q3834bjd3dlc8zsr95mvzz9n006-mesa-21.3.8
  /gnu/store/wj1nipayc74yalfmdg7wwimkfc1macbl-mesa-21.3.8

Using 'guix gc --referrers', I can see that:

- The first copy is referred to by all applications in my profile that
  link against mesa, e.g. icecat.

- The second copy is referred to directly by my profile. Indeed, I can
  see that, for example, ~/.guix-profile/lib/libGL.so is a symlink to
  /gnu/store/wj1nipayc74yalfmdg7wwimkfc1macbl-mesa-21.3.8/lib/libGL.so.

So my question boils down to: why is the copy of mesa in
~/.guix-profile/lib different to the one to which programs in the same
profile are linked? And is there any guix command I can use to inspect
exactly how/why these copies differ?

Thanks!
- Mike