Revise email-users example to handle multiple function executions for a single user

I'm using the code from the sample here:
https://github.com/firebase/functions-samples/tree/master/quickstarts/email-users

onCreate is sometimes triggered multiple times for the same user. Most of the time it is triggered once, but sometimes it is triggered two times or even 7 times (which happened yesterday).
`exports.sendWelcomeEmail = functions.auth.user().onCreate(event => {`

No errors in the log, just the normal debug statements and the text output from `console.log('New welcome email sent to:', email);`

If I choose "View all from this execution", all 7 are shown, so they come from the same execution.

7 emails are also sent, so it's not just the debug output.

Only 1 account is created.