diff --git a/lib/net/imap/authenticators.rb b/lib/net/imap/authenticators.rb
index 3659274b..e7a3af2f 100644
--- a/lib/net/imap/authenticators.rb
+++ b/lib/net/imap/authenticators.rb
@@ -7,6 +7,9 @@ module Net::IMAP::Authenticators
   # {SASL mechanism}[https://www.iana.org/assignments/sasl-mechanisms/sasl-mechanisms.xhtml]
   # implemented by +authenticator+ (for instance, <tt>"PLAIN"</tt>).
   #
+  # If +mechanism+ refers to an existing authenticator, a warning will be
+  # printed and the old authenticator will be replaced.
+  #
   # The +authenticator+ must respond to +#new+ (or #call), receiving the
   # authenticator configuration and return a configured authentication session.
   # The authenticator session must respond to +#process+, receiving the server's
@@ -14,8 +17,12 @@ module Net::IMAP::Authenticators
   #
   # See PlainAuthenticator, XOauth2Authenticator, and DigestMD5Authenticator for
   # examples.
-  def add_authenticator(auth_type, authenticator)
-    authenticators[auth_type] = authenticator
+  def add_authenticator(mechanism, authenticator, warn_overwrite: true)
+    if warn_overwrite && (original = authenticators[mechanism])
+      warn("%p: replacing %p authenticator: %p" % [ self, mechanism, original],
+           uplevel: 1)
+    end
+    authenticators[mechanism] = authenticator
   end
 
   # :call-seq: