pw: Ensure group membership is not duplicated

Fix the following problem:

1. A nonexistent user, someuser, is added to somegroup in /etc/group.
2. someuser is then created with membership in somegroup.

The entry for somegroup in /etc/group will then contain

somegroup:*:12345:someuser,someuser

With this fix, the entry will be

somegroup:*:12345:someuser

PR: 238995
Reviewed by: bapt, jrm
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41076