This behavior is normal as the constituent is receiving the sustaining gift ARs from the donation form that is tied to the TeamRaiser and they are also receiving the autoresponder from the TeamRaiser itself. Unfortunately there is no way to change this behvior, but if you want one and not the other, then you can disable the autoresponder from the TeamRaiser or the donation form. For example, if you expect to be taking sustaining gifts in your TeamRaiser event, then you can disable the Sustaining Initial Thank You and the Sustaining Subsequent Thank You. This will make sure that the constituent only receives one autoresponder per sustaining gift attempt.