From dispose doc:

The methods getSkinnable() and getNode() should return null following a call to dispose. Calling dispose twice has no effect.

The last sentence in particular implies that the calling code need not worry about duplicate calls (either by the caller itself or any other), that is the following test must pass:

    @Test
    public void testSkinDispose() {
        showControl();
        control.getSkin().dispose();
        control.getSkin().dispose();
    }
    
It fails for some skins with an NPE, f.i. Tree/TableCell, Tree/TableView (there might be others, not yet tested). It's probably one of the reaons for JDK-8243940. Extracted this because the other issue might reveal additional quirks.