This warning category is spelled [syntax.duplicate-ids] by qmllint.

Found a duplicated id

What happened?

You used the same value for different ids.

Why is this bad?

The QML language forbids duplicate ids: they should all be unique in the same QML component.

Example

import QtQuick

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

To fix this warning, rename the duplicate to become unique:

import QtQuick

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