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Distinguish drop down selection vs tapNodeData in dash cytoscape

I am developing a dashboard to visualize and explore a graph. I have a drop down selection to select a starting node and have callbacks to progressively expand the graph from this example https://github.com/plotly/dash-cytoscape/blob/master/usage-elements.py

Currently my callback looks like

@app.callback(
    Output("cytoscape-nodes", "elements"),
    [
        Input("node-selection-dropdown", "value"),
        Input('cytoscape-two-nodes', 'tapNodeData')
    ],
    [State('cytoscape-nodes', 'elements'),
     State('radio-expand', 'value')]
def filter_nodes(node, nodeData, elements, explore_mode):

Right now there is no way to figure out if a new node has been selected from the dropdown or an existing node has been selected. I get values in both node and nodeData parameters.