Hi @Sarthak S. Mohanty,
Based on your query, I understand that you would like to add schema extensions which you are able to see an extension attributes.
There are four kinds of operations to update the custom data for the resource type "user". The error you are seeing actually shows when there is a value already exist for that particular extension. If you would like to update the extension attributes, you can just make sure there is no value available for the extension attribute. You can update the values using the following document: Custom data options in Microsoft Graph.
Please do check all the four options available and try updating the data as per your requirement.I hope this information is helpful. Please feel free to reach out if you have any further questions.
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