It does not matter if a user belongs to multiple Entra ID 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 Entra ID groups that are included in backup, the user's backups are automatically stopped.
If user is removed from some Entra ID groups but still remains in at least one Entra ID group that is included in backup, the user's backups will keep running.
Note: 'Add new Entra ID Group users automatically to backup' option only impacts enabling backups for new users added to the configured Entra ID groups, but not the removal of users from these groups. This means that users removed from Entra ID groups will have their backups automatically stopped, even though the above mentioned automatic addition settings is disabled.