What happened? The verification method became invalid for many sites in the Google Search Console so Google had to re-verify those sites. During the process it re-sent verification notifications to those users.
The email read "Google has identified that [email] has been added as an owner of [domain name].”
You might want to use this is as a reminder to clean-up and manage your account; just log into your Search Console and click on the gears icon, then the 'verification details screen'.
You will see the failed attempt then the successful attempt to re-verify. Make sure that only those who should be verified for your website are and remove any that shouldn't be there.