@ras43, Thanks for posting in Q&A. For the error code "806 - ZtdDeviceAlreadyAssigned", yes, your understanding is correct. It indicates that the device is already registered to the same tenant or a record of the device already exists.
You have deleted the device in Intune, Autopilot device and need to check if it still exists in Microsoft Entra ID. We can add serial number column in Intune devices. Export it and search to see if the serial number is still existing. If yes, then we can check the device's Microsoft Entra Device ID , delete it and see if it can work.
Please try the above suggestion and if there's any update, feel free to let us know.
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