Dear Gnus,

This came up in a mu4e bug:
  https://github.com/djcb/mu/issues/2460

(for reference: mu4e is /another/ Emacs MUA, which re-uses a lot of Gnus code)

Basically, in the attached html-email there are _two_ CIDs with the same
filename (but with different content-ids)

When doing a view-in-browser, gnus-article-browse-html-save-cid-content
uses the filename in favor of the cid, and the first file will be
overwritten by the second.

Admittedly, a corner-case. But, it can be fixed.
This fixes it for me:

--8<---------------cut here---------------start------------->8---
@@ -2871,11 +2871,14 @@ gnus-article-browse-html-save-cid-content
 			      cid handle directory))
 	      (throw 'found file)))
 	   ((equal (concat "<" cid ">") (mm-handle-id handle))
-	    (setq file (or (mm-handle-filename handle)
-			   (concat
-			    (make-temp-name "cid")
-			    (car (rassoc (car (mm-handle-type handle))
-					 mailcap-mime-extensions))))
+	    ;; Files are randomized since declared filenames may not be unique.
+	    (setq file (format "cid-%d-%s"
+			       (random 99)
+			       (or (mm-handle-filename handle)
+				   (concat
+				    (make-temp-name "cid")
+				    (car (rassoc (car (mm-handle-type handle))
+						 mailcap-mime-extensions)))))
 		  afile (expand-file-name file directory))
--8<---------------cut here---------------end--------------->8---

Kind regards,
Dirk.