“use strict”; now allows duplicated properties?

Question

I just found that in the last Chrome 42 and FF 37.0.2 this lines of code are perfectly legal

"use strict";
var o = { p: 1, p: 2 };

(copy-pasted from MDN )

In IE 10-11 and Opera 28.0.1750 it throws error as expected.

In the same time,

abc=0;

causes error (undeclared variable) as expected.

Does anybody know what caused such change?

Answer 1

There is a Bugzilla ticket here. From what I gather (here and other pages I have looked up), duplicate properties are legal in ECMAScript version 6, opposed to ES5, where it is forbidden in strict mode.