I've seen this too. If you attempt to add the pool label to multiple DUTs at the same time, it always fails after adding one. But if you click the add label button again, it works with no error.

Gave a pass at trying to fix this. It seems more complicated than I thought. I tried creating some locks to make the label database calls mutually exclusive. Even with mutual exclusion, and adding ~10 seconds sleep to make sure the database transactions had finished, I was still seeing this error. 

It seems like there is something weird with the way the RPC call is handled, and the way database connections are acquired that makes it so that any simultaneous requests would fail, even with proper locking protections.

Would need to solve this on the client side, either by combining all requests into one, or serialize the requests.