Possible to override contacts for just one Nagios host group?

Solution 1:

The problem is, that when I add contact_groups sandradebug it is still only admins that gets the notification and not also sandradebug.

Have you restarted the Nagios? Your above configuration will override the template values and only sandradebug group will receive these alerts.

How can I include sandradebug for this one host, so notifications are also sent there?

Object Inheritance is what you are looking for:

define host {
    use                     linux-host
    host_name               example
    alias                   example
    address                 10.10.10.10
    hostgroups              default-linux-services
    contact_groups          +sandradebug
}

Using the + sign, the host definition uses the data in the template and adds the sandradebug to the alerts.

Solution 2:

For this, I would just combine the template with the definition since you're only using it once:

define host {
  use                     generic-host
  host_name               example
  alias                   example
  address                 10.10.10.10
  hostgroups              default-linux-services
  check_command           check-host-alive
  notification_interval   4320
  notification_options    d,u,r
  contact_groups          sandradebug,admins
}

What's likely happening is that your template is overriding the contact group specified.