bug#31680: guix system reconfigure: Inscrutable error message when adding same service twice

[Danny Milosavljevic]

I got an inscrutable error message:

------------------------------------------
[...]
`/gnu/store/xbihzbddsl12499nam1frafm3xwx0fhl-upower-0.99.4/share/dbus-1/system-services/org.freedesktop.UPower.service'
 -> 
`/gnu/store/x2vkzm0by6mlby0cwjk0ir5aycfjn9wf-upower-wrapper/share/dbus-1/system-services/org.freedesktop.UPower.service'
find-files: 
/gnu/store/xw7fwgdwmgpz75qjgq542l1m4fd8m3by-wpa-supplicant-2.6/share/dbus-1/: 
No such file or directory
Backtrace:
           3 (primitive-load "/gnu/store/n7l928agnkhyv1grpq3zn38yvqc?")
In ice-9/eval.scm:
    619:8  2 (_ #f)
In srfi/srfi-1.scm:
    640:9  1 (for-each #<procedure 902160 at ice-9/eval.scm:333:13 ?> ?)
In unknown file:
           0 (symlink "/gnu/store/5jq912zx08lbv6svv50b2mkrv75np31j-?" ?)

ERROR: In procedure symlink:
In procedure symlink: File exists
builder for 
`/gnu/store/nw48qzs8mmxq74yjnl5lhqlpxjs8hchi-dbus-system-services.drv' failed 
with exit code 1
cannot build derivation 
`/gnu/store/hh6v78zidzg11b5qk91b31hm6wvlkpjx-dbus-configuration.drv': 1 
dependencies couldn't be built
cannot build derivation `/gnu/store/ss682w5kqjazscnr7qrs7x9d3mpxhfy6-etc.drv': 
1 dependencies couldn't be built
cannot build derivation 
`/gnu/store/lx5ijdjdqkxa11p3546pi5w4pnim6j0q-system.drv': 1 dependencies 
couldn't be built
guix system: error: build failed: build of 
`/gnu/store/lx5ijdjdqkxa11p3546pi5w4pnim6j0q-system.drv' failed
------------------------------------------

This happens when you add the same service twice to config.scm .