Hi Trinadha. There's a table within the internal Tableau Postgres instance called group_users that is in sync with what AD contains in terms of membership. And it has a unique index on the group / user. So are you able to query your AD group by ldapsearch or work with your AD team? Perhaps the user is somehow entered twice for this group within AD which causes Tableau an error.
We worked with AD team and they confirm everything seems fine from their end. I will check about the users part and update you. Thanks