Hi @Sindhushree K P ,
According to this documentation 400 Bad Request response code comes when the user is already a member of the group. And I can replicate the same.
But as you mentioned the user is not the duplicated one then I would advise you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Just a thought Is it an ad synced group?
Hope this helps.
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