Check is the user is already authenticated on other site.

silent.js

window.onload = function() {

webAuth = new auth0.WebAuth({
    domain: 'saltuk-wordpress.auth0.com',
    redirectUri: 'http://dev-auth0-test1.pantheonsite.io/index.php?auth0=1',
    clientID: 'SCNHLB4vHvQlTKazudZO87LDikCyrP6s',
    responseType: 'token id_token',
});

if (document.cookie.match(/^(.*;)?\s*CheckedSessionCookie\s*=\s*[^;]+(.*)?$/) === null) {
    webAuth.checkSession({}, function (err, result){
      if (!err) {
        // There is a valid session from another app so let's redirect after setting the
        // session state checked cookie to prevent a redirect loop
        document.cookie="CheckedSessionCookie=true";
        location.href='http://dev-auth0-test1.pantheonsite.io/wp-login.php/'; 
      }
    });
  }
}