Basically I tried upgrading a room at a time my server had some lag. Not seeing any kind of feedback in Riot after a short amount of time, I ran /upgraderoom a second time, which caused two tombstone events to be sent to the same v1 room, referencing two separate v3 rooms. Additionally, the UI got confused which caused me to left the wrong room, so the upgraded room isn't joinable by anyone.

Before initiating a room upgrade, Synapse should check if there's already an upgrade ongoing for the same room, and return an error if there is.