What is the meaning of this build error message?

Resource "devices" (device) allocation error: Value was already allocated

If you have one device assigned to one blueprint (e.g., BP1) on staging, the device is still available for assignment to a different blueprint (e.g., BP2) until you commit BP1. However, when the same device is assigned to the BP2, you will see the build error on BP2 - Resource "devices" (device) allocation error: Value was already allocated. BP1 can still be committed, as BP1 has ownership of the device.