js: Avoid double declarations with let

js: Avoid double declarations with let

The following code is a syntax error in ES6:

    let a = 'something';
    let a = 'other thing';

Previously GJS would silently accept this code, but in the next release the
SpiderMonkey JS engine will be more ES6-compliant.

https://bugzilla.gnome.org/show_bug.cgi?id=778425