Allowing users to use the same email
You could use the Shared Email module.

The shared email module allows multiple users to use the same email address for different accounts. The module works for both registration and account updates.

Users using that module could also be interested in the Shared Email Password Reset module.

This module will have users reset their password based on both email and user name, there are some cases where this module can help when using the Shared Email module or when there is a requirement that passwords should be reset only if email and username matches.

Both the modules have a Drupal 8 version, and they are declared compatible with Drupal 9.