accounts: Delete duplicate entries.

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.

gnu/build/accounts.scm

@@ -238,7 +238,7 @@ to it atomically and set the appropriate permissions."
       (for-each (lambda (entry)
                   (display (entry->string entry) port)
                   (newline port))
-                entries))
+                (delete-duplicates entries)))
 
     (if (port? file-or-port)
         (write-entries file-or-port)