Component with duplicate id doesn't raise a warning if created in a callback - overwrites the callback(s) the first component was part of

[celia-lm]

Description of the issue

Screen.Recording.2025-08-29.at.13.55.06.mov

• The issue happens both if the affected callback (the one that uses the "duplicated" id as Input, Output or State) is defined with @app.callback or @dash.callback.
• suppress_callback_exceptions=True is not set in app = Dash(...).
• The rest of the callbacks (both defined with @app.callback and @dash.callback ) keep working after the component generation (not shown in the video).
• The component that the affected callback uses is the newest one.
• If, after generating the second component, I do document.getElementById in the Console, the component that's identified is the one that appears first in the html.

Case 1: sample code (original button is first)

Case 2: new button is first

Expected behavior

Component id validation should also happen when new components are added to the layout. An error like this should be shown in the debug menu or the logs:

dash.exceptions.DuplicateIdError: Duplicate component id found in the initial layout: `b3`

Code to replicate the issue

dash==3.2.0

import dash
from dash import dcc, html, Input, Output, State, callback, ctx

app = dash.Dash(__name__)

app.layout = html.Div([
    html.H3("callback 1: @app.callback, output=div1"),
    html.Button(id="b1", children="Trigger callback 1"),
    html.Div(id="div1", children="callback 1 not triggered yet"),
    ####
    html.H3("callback 2: @callback, output=div2"),
    html.Button(id="b2", children="Trigger callback 2"),
    html.Div(id="div2", children="callback 2 not triggered yet"),
    ###
    html.H3("callback 3: @app.callback, output=div3"),
    html.Button(id="b3", children="Trigger callback 3 (initial n_clicks=20)", n_clicks=20),
    html.Div(id="div3", children="callback 3 not triggered yet"),
    ####
    html.Br(),
    html.Button(id="b4", children="Generate component with duplicate id (b3)"),
    html.Br(),
    html.Div(id="div4")
])

@app.callback(
    Output("div1", "children"),
    Input("b1", "n_clicks"),
    prevent_initial_call=True
)
def update_div1_with_b1(b1):
    return f"Callback 1 triggered by {ctx.triggered_id} and {b1} clicks"

@callback(
    Output("div2", "children"),
    Input("b2", "n_clicks"),
    prevent_initial_call=True
)
def update_div1_with_b2(b2):
    return f"Callback 2 triggered by {ctx.triggered_id} and {b2} clicks"

@callback(
    Output("div3", "children"),
    Input("b3", "n_clicks"),
    prevent_initial_call=True
)
def update_div3(b3):
    return f"Callback 3 triggered by {ctx.triggered_id} and {b3} clicks"

@callback(
    Output("div4", "children"),
    Input("b4", "n_clicks"),
    prevent_initial_call=True
)
def update_div4(b4):
    return html.Button(id="b3", children="Button with duplicate id (b3)", n_clicks=0),

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