It does not matter if a user belongs to multiple AD groups that are included in backup. User's backups will only be taken once - meaning no duplicate backups.
When a user is removed from all AD groups in Microsoft 365 Azure AD that are included in backup, the user's backups are automatically stopped.
If user is removed from some AD groups but still remains in at least one AD group that is included in backup, the user's backups will keep running.
Note: 'Add new AD group users automatically to backup' option only impacts enabling backups for new users added to the configured AD groups, but not the removal of users from these groups. This means that users removed from AD groups will have their backups automatically stopped, even though the above mentioned automatic addition settings is disabled.