I am using the RadAsyncUpload in my app. So what I need as follows whenever we select any attachment to upload how to add the duplicate file checking from the client side, and that to while picking the attachments itself.
It is not possible to access the uploaded files while attaching a file in RadAsyncUpload. So one suggestion is you can access the already uploaded file names on server side and keep it in a hidden field and OnClientFileSelected event you can check the duplication using that hidden field as follows.