I checked with my logic and onboarding Email cannot be sent when there's a sending record. I think it's because the trigger activation in less than 1 second, within which time the first Email hasn't finished sending yet. So the email sending will trigger several times until the first Email sending procedure finished and a record of "Email Sent" was added.
See issue https://github.com/coderbunker/gas/issues/70
I checked with my logic and onboarding Email cannot be sent when there's a sending record. I think it's because the trigger activation in less than 1 second, within which time the first Email hasn't finished sending yet. So the email sending will trigger several times until the first Email sending procedure finished and a record of "Email Sent" was added.