Suggest alternative reason for ERT test duplication error

* lisp/emacs-lisp/ert.el (ert-set-test): Amend error message;
maybe the redefinition was caused by a file loaded twice.
(Bug#66782)

Suggested by Xiyue Deng.

(cherry picked from commit 425d23fbeaede81ab4f50b4073949cc1c8a3fbd0)

1 files changed, 1 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/ert.el b/lisp/emacs-lisp/ert.el
index be9f013ebcf..5d001307125 100644
--- a/lisp/emacs-lisp/ert.el
+++ b/lisp/emacs-lisp/ert.el
@@ -151,7 +151,7 @@ mode.")
   (when (and noninteractive (get symbol 'ert--test))
     ;; Make sure duplicated tests are discovered since the older test would
     ;; be ignored silently otherwise.
-    (error "Test `%s' redefined" symbol))
+    (error "Test `%s' redefined (or loaded twice)" symbol))
   (define-symbol-prop symbol 'ert--test definition)
   definition)