Shell: Add duplicate route validation for sibling elements

src/Controls/src/Core/Routing.cs

@@ -236,6 +236,49 @@ public static void SetRoute(Element obj, string value)
 			obj.SetValue(RouteProperty, value);
 		}
 
+		internal static void ValidateForDuplicates(Element element, string route)
+		{
+			// If setting the same route to the same element, no need to validate
+			var currentRoute = GetRoute(element);
+			if (currentRoute == route)
+			{
+				return;
+			}
+
+			// Only validate user-defined routes
+			if (string.IsNullOrEmpty(route) || !IsUserDefined(route))
+			{
+				return;
+			}
+
+			// Check for duplicate routes among siblings (elements with the same parent)
+			var parent = element.Parent;
+			if (parent == null)
+			{
+				return;
+			}
+
+			foreach (var child in parent.LogicalChildrenInternal)
+			{
+				if (child == element)
+					continue;
+
+				var siblingRoute = GetRoute(child);
+				if (siblingRoute == route)
+				{
+					throw new ArgumentException(
+						$"Duplicated Route: \"{route}\" is already registered to another element of type {child.GetType().Name}. " +
+						$"Routes must be unique among siblings to avoid navigation conflicts.",
+						nameof(route));
+				}
+			}
+		}
+
+		internal static void RemoveElementRoute(Element element)
+		{
+			// No longer needed with sibling-based validation, but keep for API compatibility
+		}
+
 		static void ValidateRoute(string route, RouteFactory routeFactory)
 		{
 			if (string.IsNullOrWhiteSpace(route))

src/Controls/src/Core/Shell/BaseShellItem.cs

@@ -103,7 +103,11 @@ public bool IsEnabled
 		public string Route
 		{
 			get { return Routing.GetRoute(this); }
-			set { Routing.SetRoute(this, value); }
+			set
+			{
+				Routing.ValidateForDuplicates(this, value);
+				Routing.SetRoute(this, value);
+			}
 		}
 
 		/// <include file="../../../docs/Microsoft.Maui.Controls/BaseShellItem.xml" path="//Member[@MemberName='Title']/Docs/*" />

src/Controls/tests/Core.UnitTests/ShellTests.cs

@@ -1674,5 +1674,123 @@ public void ShellContentTitleShouldNotBeAppliedMultipleTimesWithStringFormat()
 
 			Assert.Equal("Title: Hello, World!", shellContent.Title);
 		}
+
+		[Fact]
+		public void DuplicateSiblingRoutesShouldThrowArgumentException()
+		{
+			var shell = new Shell();
+			var sameRoute = "DuplicateRoute";
+
+			var flyoutItem = new FlyoutItem();
+			var shellSection = new ShellSection();
+			var shellContent1 = new ShellContent { Title = "Page1", Content = new ContentPage() };
+			var shellContent2 = new ShellContent { Title = "Page2", Content = new ContentPage() };
+			shellSection.Items.Add(shellContent1);
+			shellSection.Items.Add(shellContent2);
+			flyoutItem.Items.Add(shellSection);
+			shell.Items.Add(flyoutItem);
+
+			shellContent1.Route = sameRoute;
+			var exception = Assert.Throws<ArgumentException>(() =>
+			{
+				shellContent2.Route = sameRoute;
+			});
+
+			Assert.Equal($"Duplicated Route: \"{sameRoute}\" is already registered to another element of type ShellContent. Routes must be unique among siblings to avoid navigation conflicts. (Parameter 'route')", exception.Message);
+		}
+
+		[Fact]
+		public void SameRouteInDifferentParentsIsAllowed()
+		{
+			var shell = new Shell();
+			var sameRoute = "SharedRoute";
+
+			// Create two different ShellSections, each with their own ShellContent
+			var flyoutItem = new FlyoutItem();
+			var shellSection1 = new ShellSection();
+			var shellSection2 = new ShellSection();
+			var shellContent1 = new ShellContent { Title = "Page1", Content = new ContentPage() };
+			var shellContent2 = new ShellContent { Title = "Page2", Content = new ContentPage() };
+
+			shellSection1.Items.Add(shellContent1);
+			shellSection2.Items.Add(shellContent2);
+			flyoutItem.Items.Add(shellSection1);
+			flyoutItem.Items.Add(shellSection2);
+			shell.Items.Add(flyoutItem);
+
+			// Both should be able to have the same route since they're in different parents
+			shellContent1.Route = sameRoute;
+			shellContent2.Route = sameRoute; // Should not throw - different parents
+
+			Assert.Equal(sameRoute, shellContent1.Route);
+			Assert.Equal(sameRoute, shellContent2.Route);
+		}
+
+		[Fact]
+		public void ChangingRouteAllowsReuseAmongSiblings()
+		{
+			var shell = new Shell();
+			var route = "TestRoute";
+
+			var flyoutItem = new FlyoutItem();
+			var shellSection = new ShellSection();
+			var shellContent1 = new ShellContent { Title = "Page1", Content = new ContentPage() };
+			var shellContent2 = new ShellContent { Title = "Page2", Content = new ContentPage() };
+			shellSection.Items.Add(shellContent1);
+			shellSection.Items.Add(shellContent2);
+			flyoutItem.Items.Add(shellSection);
+			shell.Items.Add(flyoutItem);
+
+			// Set initial route
+			shellContent1.Route = route;
+
+			// Change the route to something else
+			shellContent1.Route = "NewRoute";
+
+			// Now the original route should be available for the sibling
+			shellContent2.Route = route; // Should not throw
+			Assert.Equal(route, shellContent2.Route);
+		}
+
+		[Fact]
+		public void RemovingElementClearsRoute()
+		{
+			var shell = new Shell();
+			var route = "RemovableRoute";
+
+			var flyoutItem = new FlyoutItem();
+			var shellSection = new ShellSection();
+			var shellContent1 = new ShellContent { Title = "Page1", Content = new ContentPage() };
+			var shellContent2 = new ShellContent { Title = "Page2", Content = new ContentPage() };
+			shellSection.Items.Add(shellContent1);
+			shellSection.Items.Add(shellContent2);
+			flyoutItem.Items.Add(shellSection);
+			shell.Items.Add(flyoutItem);
+
+			shellContent1.Route = route;
+
+			// Remove the element from ShellSection.Items
+			shellSection.Items.Remove(shellContent1);
+
+			// Now the route should be available for another element
+			shellContent2.Route = route; // Should not throw
+			Assert.Equal(route, shellContent2.Route);
+		}
+
+		[Fact]
+		public void ReassigningSameRouteToSameElementDoesNotThrow()
+		{
+			var shell = new Shell();
+			var route = "SameRoute";
+
+			var flyoutItem = new FlyoutItem();
+			var shellContent = new ShellContent { Title = "Page" };
+			flyoutItem.Items.Add(shellContent);
+			shell.Items.Add(flyoutItem);
+
+			shellContent.Route = route;
+			shellContent.Route = route; // Should not throw - same element, same route
+			Assert.Equal(route, shellContent.Route);
+		}
 	}
 }

src/Controls/tests/TestCases.HostApp/Issues/XFIssue/Issue6878.cs

@@ -28,7 +28,6 @@ protected override void Init()
 
 		CurrentItem = Items.Last();
 
-		AddTopTab(TopTab);
 		AddBottomTab("Bottom tab");
 	}