Add the UniqueEntity attribute to add an application-level validation.

#[UniqueEntity(fields: ['email', 'group'], message: 'A user with this email already exists in this group')]
class User {
  
}

Add the ORM\UniqueConstraint attribute to add a database-level unique constraint.

#[UniqueEntity(fields: ['email'], message: 'A user with this email already exists')]
#[ORM\UniqueConstraint(
    name: 'email_group_unique',
    columns: ['email', 'group_id']
)]
class User {
  #[ORM\Column(length: 255, unique: true)]
  private ?string email = null;
}