When adding multiple instances of a service requiring some user
account/group, we could end up with multiple entries for that account or
group in /etc/passwd or /etc/group.
* gnu/build/accounts.scm (database-writer)[write-entries]: Add call to
'delete-duplicates'.
* tests/accounts.scm ("write-passwd with duplicate entry"): New test.
This is a followup to 504a0fc636ec591e65b4a229a37e522e425d8a0c.
* tests/accounts.scm ("allocate-passwd with previous state"): Change
expected 'shell' for "alice" to "/bin/sh".
