A redirect response returns an HTTP status code of
3xx
along with a header telling the browser which URL should be requested.
You cannot combine that with a response body that shows an alert message.
If you want to show an alert before redirecting, you'll need to use script to redirect:
return View("AlertAndRedirect");
AlertAndRedirect.cshtml
<script>
alert("successfully updated");
location.assign('@Url.Action("List")');
</script>