There are two posts on the official forums explaining this.
https://forum.sophgo.com/t/use-opensbi-to-boot-your-own-operating-system/340
And I made a youtube video with the steps https://youtu.be/UrKmSipiXpc?si=H8WHHjq7kc-XSJHM
The Duo expects a fip.bin file which combines a bl2.bin (generated from the sdk) and a bl33.bin (this is the user application).
The SDK has a tool to combine them.
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