From e190768d7ae3db400e744d6d1d594969aacd2bf9 Mon Sep 17 00:00:00 2001
From: Philippe Duval <philippe@plot.ly>
Date: Tue, 31 Jul 2018 13:31:10 -0400
Subject: [PATCH] Disallow duplicate component ids.

---
 dash/dash.py       | 25 ++++++++++++++++++-------
 dash/exceptions.py |  4 ++++
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/dash/dash.py b/dash/dash.py
index 32d6ee1b22..84bad37263 100644
--- a/dash/dash.py
+++ b/dash/dash.py
@@ -843,13 +843,8 @@ def dispatch(self):
 
         return self.callback_map[target_id]['callback'](*args)
 
-    def _setup_server(self):
-        if self.config.include_assets_files:
-            self._walk_assets_directory()
-
-        # Make sure `layout` is set before running the server
-        value = getattr(self, 'layout')
-        if value is None:
+    def _validate_layout(self):
+        if self.layout is None:
             raise exceptions.NoLayoutException(
                 ''
                 'The layout was `None` '
@@ -857,6 +852,22 @@ def _setup_server(self):
                 'Make sure to set the `layout` attribute of your application '
                 'before running the server.')
 
+        layout_id = getattr(self.layout, 'id', None)
+
+        component_ids = {layout_id} if layout_id else set()
+        for component in self.layout.traverse():
+            component_id = getattr(component, 'id', None)
+            if component_id and component_id in component_ids:
+                raise exceptions.DuplicateIdError(
+                    'Duplicate component id found : `{}`'.format(component_id))
+            component_ids.add(component_id)
+
+    def _setup_server(self):
+        if self.config.include_assets_files:
+            self._walk_assets_directory()
+
+        self._validate_layout()
+
         self._generate_scripts_html()
         self._generate_css_dist_html()
 
diff --git a/dash/exceptions.py b/dash/exceptions.py
index c459b0e5c2..58cf322715 100644
--- a/dash/exceptions.py
+++ b/dash/exceptions.py
@@ -50,5 +50,9 @@ class PreventUpdate(CallbackException):
     pass
 
 
+class DuplicateIdError(DashException):
+    pass
+
+
 class InvalidCallbackReturnValue(CallbackException):
     pass