diff -r 29cedaaa8cbd tex.el
--- a/tex.el	Thu Aug 31 23:39:35 2017 +0900
+++ b/tex.el	Fri Sep 01 16:55:26 2017 +0900
@@ -914,12 +914,11 @@
   "Return a list of all elements in ALIST, but each car only once.
 Elements of KEEP-LIST are not removed even if duplicate."
   ;; Copy of `reftex-uniquify-by-car' (written by David Kastrup).
-  (setq keep-list (sort (copy-sequence keep-list) #'string<))
+  (setq keep-list (TeX-sort-strings keep-list))
   (setq alist (sort (copy-sequence alist)
-		    (lambda (a b)
-		      (string< (car a) (car b)))))
+		    #'TeX-car-string-lessp))
   (let ((new alist) elt)
-    (while new
+    (while (cdr new)
       (setq elt (caar new))
       (while (and keep-list (string< (car keep-list) elt))
 	(setq keep-list (cdr keep-list)))
@@ -933,7 +932,7 @@
   "Return a list of all strings in LIST, but each only once."
   (setq list (TeX-sort-strings list))
   (let ((new list) elt)
-    (while new
+    (while (cdr new)
       (setq elt (car new))
       (while (string= elt (cadr new))
 	(setcdr new (cddr new)))
@@ -944,6 +943,11 @@
   "Return sorted list of all strings in LIST."
   (sort (copy-sequence list) #'string<))
 
+(defun TeX-car-string-lessp (s1 s2)
+  "Compare the cars of S1 and S2 in lexicographic order.
+Return t if first is less than second in lexicographic order."
+  (string-lessp (car s1) (car s2)))
+
 ;;; Buffer
 
 (defgroup TeX-output nil
@@ -4737,11 +4741,6 @@
 ;; Some of these functions has little to do with TeX, but nonetheless we
 ;; should use the "TeX-" prefix to avoid name clashes.
 
-(defun TeX-car-string-lessp (s1 s2)
-  "Compare the cars of S1 and S2 in lexicographic order.
-Return t if first is less than second in lexicographic order."
-  (string-lessp (car s1) (car s2)))
-
 (defun TeX-listify (elt)
   "Return a newly created list with element ELT.
 If ELT already is a list, return ELT."