Sharing in case anyone else gets stuck here - I setup a simple business event aimed at triggering an action. Each time I tried to setup the Action Execution I would get an error reading Error: Another process has added the 'ActionExecution' record. Your changes will be lost.
After numerous attempts I noticed in the trace log that it also stated Cannot insert duplicate key row in object 'dbo.ActionExecution' with unique index 'ActionExecution_Name'. which made me finally realized that we had an old Action Execution with the same SubscriberID in a test tenant
This wasn’t obvious, since both the Business event and Action Execution were new and totally unique, but after trying a new name on the subscriber ID I was able to save successfully