Issue originally reported by @danez in babel/babylon#78

According to the spec when defining multiple variables with the same name, the parser should throw and SyntaxError when using let or const:

let foo, foo;
or
let foo; let foo;

From 13.2.1.1:

It is a Syntax Error if the BoundNames of BindingList contains any duplicate entries.

V8 trows an error for let and const, but not vor var.