Delete duplicated users on Singup

function (user, context, callback) {
    var ManagementClient = require("[email protected]").ManagementClient;

    var mgt = new ManagementClient({
        domain: "{auth0 domain}",
        clientId: "{client ID}",
        clientSecret: "{Client secret}",
        audience: "https://{auth0 domain}/api/v2/"
    });

    mgt.getUsersByEmail(user.email).then(res => {
        console.log("checking user exist or not");
        if (res.length >= 1 || res !== null || res !== "") {
           console.log(user.user_id);
            mgt.deleteUser({id:user.user_id}).then(res => {
              console.log("Deleting user");
                console.log(res);
                return callback(new UnauthorizedError("Access Denied - An account already exist"));
            }).catch(err => {
                console.log("Failed to delete user");
                console.log(err);
            });
        }
    }).catch(err => {
        console.log(err);
    });

    callback(null, user, context);
}