[Blazor] Throw error when custom event name matches browserEventName to prevent double-triggering (#64774)

Author: Copilot

src/Components/Web.JS/src/Rendering/Events/EventTypes.ts

@@ -23,6 +23,12 @@ export function registerCustomEventType(eventName: string, options: EventTypeOpt
     throw new Error(`The event '${eventName}' is already registered.`);
   }
 
+  // When aliasing a browser event, the custom event name must be different from the browser event name
+  // to avoid double-triggering (once for the browser event, once for the custom event wrapper)
+  if (options.browserEventName && eventName === options.browserEventName) {
+    throw new Error(`The custom event '${eventName}' cannot have the same name as its browserEventName '${options.browserEventName}'. Choose a different name for the custom event.`);
+  }
+
   // If applicable, register this as an alias of the given browserEventName
   if (options.browserEventName) {
     const aliasGroup = browserEventNamesToAliases.get(options.browserEventName);

src/Components/test/E2ETest/Tests/EventCustomArgsTest.cs

@@ -170,6 +170,17 @@ public void CanRegisterCustomEventAndSupplyComplexParams()
             line => Assert.Equal("Event with IJSObjectReference received: Hello!", line));
     }
 
+    [Fact]
+    public void ThrowsErrorWhenEventNameMatchesBrowserEventName()
+    {
+        // Attempt to register a custom event with the same name as the browser event
+        Browser.Exists(By.Id("register-invalid-same-name-event")).Click();
+        
+        // Verify that an error is thrown
+        var errorMessage = Browser.Exists(By.Id("same-name-event-error")).Text;
+        Assert.Contains("cannot have the same name as its browserEventName", errorMessage);
+    }
+
     void SendKeysSequentially(IWebElement target, string text)
     {
         foreach (var c in text)

src/Components/test/testassets/BasicTestApp/EventCustomArgsComponent.razor

@@ -60,6 +60,15 @@
             onclick="Blazor.registerCustomEventType('custommouseover', { browserEventName: 'mouseover' })">
         Register custom mouseover event (which has no corresponding native listener)
     </button>
+
+    <button id="register-invalid-same-name-event"
+            onclick="try { Blazor.registerCustomEventType('scrolltop', { browserEventName: 'scrolltop' }); document.getElementById('same-name-event-error').textContent = 'No error thrown'; } catch (error) { document.getElementById('same-name-event-error').textContent = error.message; }">
+        Try to register event with same name as browserEventName (should throw error)
+    </button>
+</p>
+
+<p>
+    <span id="same-name-event-error"></span>
 </p>
 
 <p>