Consider 

import QtQuick

Item {
    id: root
    Item {
        id: root
    }
}

qmllint will not warn about the duplicate id. Furthermore, when accessing properties that exist only one one of the types (and the "wrong" type is picked internally), we end up with rather confusing error messages:

import Qt
QuickItem {
    id: root
    property var s
    Rectangle {
        id: root
        property var t: root.s
    }
}

will result in the not so obvious

Warning: test.qml:9:30: Property "s" not found on type "Rectangle"
{{ property var t: root.s}}