I don't understand this Javascript error : <p>I'm getting this error when using sample code from the up and going book in the ydkjs series. Can anyone help me out? The error is </p>

<p>Uncaught SyntaxError: Identifier 'ACCESSORY_PRICE' has already been declared
    at :1:1</p>

<pre><code>const ACCESSORY_PRICE = 9.99;
var bank_balance = 302.13;
var amount = 99.99;

amount = amount * 2;

// can we afford the extra purchase?
if ( amount &lt; bank_balance ) {
    console.log( "I'll take the accessory!" );
    amount = amount + ACCESSORY_PRICE;
}
// otherwise:
else {
    console.log( "No, thanks." );
}
</code></pre>
