Did you check, if you added up a second email address when setting up your Apple Account?
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Primary email address
In most cases, what you use to sign in to your Apple Account is also the primary email address of your account. This is the email address that receives communications related to your Apple Account.
Change your Apple Account primary email address
Additional email addresses
Add extra email addresses to help friends and family find you on Apple services like FaceTime, Messages, Photo Sharing, and Find My. These email addresses can also be used to sign in to your Apple Account.
Add or delete additional email addresses
Rescue email address
If you don’t use two-factor authentication, you might have the option to add a rescue email address to your account for more security. Apple uses this to send you important account and security related information, if you have this.
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copied from: About your Apple Account email addresses - Apple Support
To find out which additional email addresses you are using, go to Settings -> Your Name -> Sign-In & Security -> Email & Phone Numbers.