diff --git a/lisp/textmodes/flyspell.el b/lisp/textmodes/flyspell.el
index 4d7a18969e..cf209abf6a 100644
--- a/lisp/textmodes/flyspell.el
+++ b/lisp/textmodes/flyspell.el
@@ -68,6 +68,11 @@ flyspell-mark-duplications-flag
   :group 'flyspell
   :type 'boolean)
 
+(defcustom flyspell-case-fold-duplications t
+  "Non-nil means Flyspell matches duplicate words case-insensitively."
+  :group 'flyspell
+  :type 'boolean)
+
 (defcustom flyspell-mark-duplications-exceptions
   '((nil . ("that" "had")) ; Common defaults for English.
     ("\\`francais" . ("nous" "vous")))
@@ -1150,7 +1155,8 @@ flyspell-word
 			      (- (save-excursion
                                    (skip-chars-backward " \t\n\f")))))
 			  (p (when (>= bound (point-min))
-			       (flyspell-word-search-backward word bound t))))
+			       (flyspell-word-search-backward
+                                word bound flyspell-case-fold-duplications))))
 		     (and p (/= p start)))))
 	    ;; yes, this is a doublon
 	    (flyspell-highlight-incorrect-region start end 'doublon)