diff --git a/lisp/emacs-lisp/map.el b/lisp/emacs-lisp/map.el
index 77431f0c59..9db2ddd95f 100644
--- a/lisp/emacs-lisp/map.el
+++ b/lisp/emacs-lisp/map.el
@@ -202,6 +202,25 @@ map-nested-elt
                   map)
       default))
 
+(defun map--remove-duplicate-keys (map)
+  "Make a version of MAP without duplicated keys.
+
+Unlike hash tables, association and property lists allow
+duplicate keys in the data structure.  This function returns the
+map with such keys and their respective data removed."
+  (if (map--plist-p map)
+      (let ((found-keys)
+            (result))
+        (while map
+          (let ((key (pop map))
+                (value (pop map)))
+            (unless (member key found-keys)
+              (push key found-keys)
+              (push key result)
+              (push value result))))
+        (nreverse result))
+    (seq-uniq map (lambda (a b) (equal (car a) (car b))))))
+
 (cl-defgeneric map-keys (map)
   "Return the list of keys in MAP.
 The default implementation delegates to `map-apply'."
@@ -234,6 +253,7 @@ map-length
   (hash-table-count map))
 
 (cl-defmethod map-length ((map list))
+  (setq map (map--remove-duplicate-keys map))
   (if (map--plist-p map)
       (/ (length map) 2)
     (length map)))
@@ -489,7 +509,7 @@ map-apply
       (cl-call-next-method)
     (mapcar (lambda (pair)
               (funcall function (car pair) (cdr pair)))
-            map)))
+            (map--remove-duplicate-keys map))))
 
 (cl-defmethod map-apply (function (map hash-table))
   (let (result)
@@ -504,6 +524,7 @@ map-apply
                    map))
 
 (cl-defmethod map-do (function (map list))
+  (setq map (map--remove-duplicate-keys map))
   (if (map--plist-p map)
       (while map
         (funcall function (pop map) (pop map)))