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pam_unix(8) does not return PAM_USER_UNKNOWN if the user does not exist in the local passwd(5)/shadow(5) files.  Therefore it's impossible to compose predicates in the pam.conf(5) file that involve them.

For example, suppose I want to compose a set of auth rules that look like the following, to enable 2-factor authentication (using pam_duo), whether the user is in the local authentication database or in LDAP:

auth        required      pam_env.so
auth        [user_unknown=ignore default=die] pam_unix.so nullok try_first_pass
auth        requisite     pam_ldap.so use_first_pass ignore_unknown_user
auth        requisite     pam_succeed_if.so uid >= 1000 quiet_success
auth        sufficient    pam_duo.so
auth        required      pam_deny.so

The Debian team enables this behavior already; see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=204506

I do not think changing this behaviour of pam_unix in a released RHEL version is appropriate. Users might have already configurations that depend on this current behaviour.
Also there is pam_localuser which purpose is exactly that.

Thanks for pointing me to pam_localuser.  Here's a solution that appears to work:

auth        required      pam_env.so
# Don't consult local password db (skip next statement) if user is unknown
auth        [user_unknown=1] pam_localuser.so
# If user is known in local password db, and validation fails, fail completely.
# If validation succeeds, skip LDAP validation.
auth        [success=1 new_authtok_reqd=ok ignore=ignore default=die]    pam_unix.so nullok try_first_pass
auth        requisite     pam_ldap.so use_first_pass
auth        requisite     pam_succeed_if.so uid >= 1000 quiet_success
auth        sufficient    pam_duo.so
auth        required      pam_deny.so