It'll probably be fixed in the next firmware update, but some folks have figured out a swap trick to get the older Everdrives running on the Duo. I tested it out with my V1.2 this evening and enjoyed some Santatlantean!
For those with the new Duo and an older Everdrive, here's how it's done.
1. Load up any HuCARD. All you need to do is be on the main title screen of a game.
2. Bring up Analogue's menu and quit the game.
3. Go to the Library and select the game you were just playing so it shows the details page for that game.
4. While on this screen, remove the HuCARD from the Duo and plug in your Everdrive.
5. From the game details page, start the game you were last playing.
It will now load into the Everdrive instead.
Using this method, I wasn't able to boot up Supergafx games that were on my Everdrive. Maybe I'd need a SGX game first to boot into for the Duo to load SGX games.
Anyway, this opens things up for folks who don't have a large library of HuCARDs.