Error message needed if ids are not unique in multi-page apps

Currently if id's in multi-page apps are not unique, and they are used as an input in a callback, the callback fails silently.

In the MWE below, it's obvious what the problem is, but in a large app, this can be difficult to debug without an error message. Only the last instance of the component with the duplicate id will trigger the callback. If you intended the callback to control the first input - everything about the callback appears to be correct. When it doesn't fire, you just need to know to look for a duplicate id somewhere in the app.

@alexcjohnson suggested the error could be caught here: https://github.com/plotly/dash/blob/dev/dash/dash-renderer/src/actions/paths.js#L23

- app.py
- pages   
   |-- page1.py
   |-- page2.py

page1.py

from dash import  html, dcc

layout = html.Div(
    [
        dcc.Link('Go to App 2', href='/pages/page2'),
        html.Div("Hello Multi-page app")
    ]
)

page2.py

from dash import Dash, dcc, html, Output, Input, callback

layout = html.Div([
    dcc.Link('Go to App 1', href='/pages/page1'),
    dcc.Input(id='input1'),
    dcc.Input(id='input1'),
    html.Div(id='output2')
])

@callback(Output('output2', 'children'), Input('input1', 'value'))
def update(value):
    return f'you have entered {value}'

app.py

from dash import Dash, dcc, html, Input, Output, callback
from pages import page1, page2

app = Dash(__name__, suppress_callback_exceptions=True)

app.layout = html.Div([
    dcc.Location(id='url', refresh=False),
    html.Div(id='page-content')
])


@app.callback(Output('page-content', 'children'),
              Input('url', 'pathname'))
def display_page(pathname):
    if pathname == '/pages/page1':
        return page1.layout
    elif pathname == '/pages/page2':
        return page2.layout
    elif pathname == '/':
        return page1.layout
    else:
        "404"

if __name__ == '__main__':
    app.run_server(debug=True)

Start with the duplicate-id callback behavior described in the MWE and inspect dash-renderer/src/actions/paths.js at the linked location. Reproduce the multi-page example, then determine how the renderer should report duplicate IDs; done means the silent failure produces a useful error message for the affected callback.