[exim] EXIM Authenticator Question

(1) The following block of text to the authenticators section in
exim.conf:

plain:
driver = plaintext
public_name = PLAIN
server_prompts = :
server_set_id = $2
server_condition = "${if pam{$2:$3}{1}{0}}"

login:
driver = plaintext
public_name = LOGIN
server_prompts = "userID:: : password::"
server_set_id = $1
server_condition = "${if pam{$1:$2}{1}{0}}"


(2) Yields an error message from a syntax check:
two server authenticators (courier_plain and plain) have the same public
name(PLAIN)

(3) Question: What is the correct authenticator script to remove the
syntax error message?