Emacs: Delete history duplicate (in Helm, among others).

.emacs.d/lisp/main.el

@@ -38,6 +38,10 @@
  url-history-track t
  url-history-file (expand-file-name "url/history" user-emacs-directory)
 
+ ;; History duplicates are useless (?) and annoying with recent Helm:
+ ;; https://github.com/syl20bnr/spacemacs/issues/13564
+ history-delete-duplicates t
+
  ;; Timeout before echoing the prefix of an unfinished keystroke.
  echo-keystrokes 0.5