Need syntax error for duplicate lexical variable declarations

[GoogleCodeExporter]

    let x;
    let x;

and

    const x = 5;
    const x = 10;

and

    let x;
    const x = 5;

should all give syntax errors.

Original issue reported on code.google.com by dome...@domenicdenicola.com on 4 Mar 2013 at 6:13