The og:url meta tag on blog posts is set to https://oppi.li, so when I save the post https://oppi.li/posts/tales_from_mainframe_modernization/ in Instapaper, the canonical URL becomes https://oppi.li instead.
I think the og:url tags should be https://oppi.li/posts/tales_from_mainframe_modernization/ instead.
thanks for the report, this should be fixed now!