`circular = false` changes behavior for objects with duplicate, but non-circular references

[nathanfdunn]

var clone = require('clone');
var obj = {}
var obj2 = { a: obj, b: obj };
var noFlagClone = clone(obj2);
console.log( noFlagClone.a === noFlagClone.b );     //true
var withFlagClone = clone(obj2, false);
console.log( withFlagClone.a === withFlagClone.b ); //false