Apache Rewrite rules - remove '/'s in URL
Something like this should do what you want:
RewriteRule ^(/~dun3/archives/).+?/([^/]+/[^/]+)$ $1$2 [L,R=301]
(The final R=301 part sends a HTTP 301 Moved Permanently header, which I presume is what you want to do, but if not you can simply omit that part.)