diff --git a/lisp/emacs-lisp/package.el b/lisp/emacs-lisp/package.el
index 84b6987a21a..a65564151bc 100644
--- a/lisp/emacs-lisp/package.el
+++ b/lisp/emacs-lisp/package.el
@@ -1463,9 +1463,9 @@ This populates `package-archive-contents'.  If ASYNC is non-nil,
 perform the downloads asynchronously."
   ;; The downloaded archive contents will be read as part of
   ;; `package--update-downloads-in-progress'.
-  (setq package--downloads-in-progress
-        (append package-archives
-                package--downloads-in-progress))
+  (dolist (archive package-archives)
+    (cl-pushnew archive package--downloads-in-progress
+                :test #'equal))
   (dolist (archive package-archives)
     (condition-case-unless-debug nil
         (package--download-one-archive