docs: runtime error if setting the same event listener 2x rather than silent failure (#2383)

Delegated event listeners do not support adding more than one event listener of the same type. This can cause confusion if two listeners are added, as one is silently dropped.

leptos_dom/src/events.rs

@@ -70,6 +70,12 @@ pub fn add_event_listener<E>(
 
     let cb = Closure::wrap(cb as Box<dyn FnMut(E)>).into_js_value();
     let key = intern(&key);
+    debug_assert_eq!(
+        Ok(false),
+        js_sys::Reflect::has(target, &JsValue::from_str(&key)),
+        "Error while adding {key} event listener, a listener of type {key} \
+         already present."
+    );
     _ = js_sys::Reflect::set(target, &JsValue::from_str(&key), &cb);
     add_delegated_event_listener(&key, event_name, options);
 }