All firmware is the latest versions from GitHub.
The fact that both the card supplied with the Duet, and a known good spare both exhibit the same problem would probably excuse the card, but I will give it a try with a brand new, straight out of the packet card, just in case. The Duet is certainly not having any trouble reading from either card as I'm having no problems printing files from them.
The router is no more than 2.5m from the Duet, which has the WiFi antenna sticking up out of a slot in its plastic housing. The devices are in sight of each other, so If -60db is too low then I suspect there might be something wrong with the Duet's WiFi board. I'll try removing the Duet from the case. I don't suspect green PLA to have any strange properties like completely de-tuning the WiFi board's antenna, but you never know with microwaves! 🙂