Adding duplicate element by childList.add

[suresh-gangumalla]

Adding an element that is already exists in childList through childList.add(Element) and childList.addAt(Element, childlist.length) Leads to throwing an error

It seems the #314 fix introduced this issue. This PR should resolve #311 and #509

`
With this fix the following scenarios are verified

 // Adding existing Element to childList through add()
childList.add(Element)

// Adding existing element to childList through addAt()
childList.addAt(Element, childList.length)

// Set existing element at end of chidList
childList.setAt(Element, childList.length)

// Set new Element at end of childList
childList.setAt(newElement, childList.length)
    // This statement should throw Error as new element can't be replaced at non existing index

// Set new Element at existing index location
childList.setAt(newElement, 0)`

[wouterlucas]

LGTM