Duplicate the layer with **layerName**, assign
        **newLayerName** to the new layer and insert the
        new layer into the font. ::

            >>> layer = font.duplicateLayer("layer 1", "layer 2")

def duplicateLayer(self, layerName, newLayerName):
        """
        Duplicate the layer with **layerName**, assign
        **newLayerName** to the new layer and insert the
        new layer into the font. ::

            >>> layer = font.duplicateLayer("layer 1", "layer 2")
        """
        layerOrder = self.layerOrder
        layerName = normalizers.normalizeLayerName(layerName)
        if layerName not in layerOrder:
            raise ValueError("No layer with the name '%s' exists." % layerName)
        newLayerName = normalizers.normalizeLayerName(newLayerName)
        if newLayerName in layerOrder:
            raise ValueError("A layer with the name '%s' already exists." % newLayerName)
        newLayer = self._duplicateLayer(layerName, newLayerName)
        newLayer = normalizers.normalizeLayer(newLayer)
        return newLayer