As reported by sss2 on IRC [1]. I can see this has changed recently, I
don't know why [2].

  guix package -i pidgin spice-gtk --no-substitutes
  The following package will be upgraded:
     spice-gtk (dependencies or package changed)
  
  The following package will be installed:
     pidgin 2.14.1
  
  The following derivations will be built:
     /gnu/store/a4n2ankaiqv857dbaqj82bmmk6bmd6ha-spice-gtk-0.37.drv
     /gnu/store/jh9z9mn9jz6744fyb3g3q5cn2nhhi65z-spice-gtk-0.37.tar.bz2.drv
  
  building /gnu/store/jh9z9mn9jz6744fyb3g3q5cn2nhhi65z-spice-gtk-0.37.tar.bz2.drv...
  downloading from https://spice-space.org/download/gtk/spice-gtk-0.37.tar.bz2 ...
  building /gnu/store/a4n2ankaiqv857dbaqj82bmmk6bmd6ha-spice-gtk-0.37.drv...
  guix package: error: profile contains conflicting entries for gtk+
  guix package: error:   first entry: gtk+@3.24.23 /gnu/store/1biqkwwsx20rbg8xm1y66qdj41h2jp4h-gtk+-3.24.23
  guix package: error:    ... propagated from spice-gtk@0.37
  guix package: error:   second entry: gtk+@2.24.32 /gnu/store/19av64kqzb9y9d9lfhqfy6bb4xl8z0zh-gtk+-2.24.32
  guix package: error:    ... propagated from pidgin@2.14.1
  hint: Try upgrading both `spice-gtk' and `pidgin', or remove one of them from the profile.


1: http://logs.guix.gnu.org/guix/2021-01-04.log#212727
2: https://git.savannah.gnu.org/cgit/guix.git/commit/?id=3fd4477809caaeb6681999941f21baac14d87e84