Fix delete-duplicate-lines

* lisp/sort.el (delete-duplicate-lines): Delete duplicate
first line when operating backward (bug#23863).

lisp/sort.el

@@ -596,7 +596,7 @@ is non-nil, it also prints a message describing the number of deletions."
 	   (equal current-prefix-arg '(64))
 	   t)))
   (let ((lines (unless adjacent (make-hash-table :test 'equal)))
-	line prev-line
+	line prev-line first-line
 	(count 0)
 	(beg (copy-marker beg))
 	(end (copy-marker end)))
@@ -604,8 +604,9 @@ is non-nil, it also prints a message describing the number of deletions."
       (goto-char (if reverse end beg))
       (if (and reverse (bolp)) (forward-char -1))
       (while (if reverse
-		 (and (> (point) beg) (not (bobp)))
+		 (not first-line)
 	       (and (< (point) end) (not (eobp))))
+	(setq first-line (and reverse (or (<= (point) beg) (bobp))))
 	(setq line (buffer-substring-no-properties
 		    (line-beginning-position) (line-end-position)))
         (if (and keep-blanks (string= "" line))