How to handle error returned from username duplicate using the unique constraint?

Here's my User database schema:

create table users (
    id serial primary key,
    name text NOT NULL,
    username text UNIQUE NOT NULL,
    password text NOT NULL,
    admin_access boolean default 'FALSE'
);

Here's where I add a new user:

    static add(name, username, password) {
        return db.one(
            `insert into users
            (name, username, password)
            values
            ($1, $2, $3)
            returning id` , [name, username, password]
        )
        .then(data => {
            return new User (data.id, name, username, password)
        })
    }

Here's where the form data is taken and I call User.add:

app.post('/addUser', protectRoute, (req, res) => {
    const newName = req.body.name;
    const newUsername = req.body.username;
    const newPassword = req.body.password;

    User.add(newName, newUsername, newPassword)
        .catch(() => {
            res.redirect(`/addUser`);
        })
        .then(newUser => {
            req.session.user = newUser;
            res.redirect(`users`);
        })
})

The error I get when I add a new user is: UnhandledPromiseRejectionWarning: Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

I'm not quite sure how to handle the error returned and give a message or alert of some kind that tells the admin that the username already exists