Prevent controls appearing twice in logical tree.

src/Perspex.Controls/Mixins/ContentControlMixin.cs

@@ -74,7 +74,7 @@ namespace Perspex.Controls.Mixins
                         UpdateLogicalChild(
                             sender,
                             logicalChildren,
-                            null,
+                            logicalChildren.FirstOrDefault(),
                             presenter.GetValue(ContentPresenter.ChildProperty));
 
                         subscriptions.Value.Add(sender, subscription);

tests/Perspex.Controls.UnitTests/ContentControlTests.cs

@@ -98,6 +98,24 @@ namespace Perspex.Controls.UnitTests
             Assert.Equal(new[] { child }, target.GetLogicalChildren());
         }
 
+        [Fact]
+        public void Control_Content_Should_Be_Logical_Child_After_ApplyTemplate()
+        {
+            var target = new ContentControl
+            {
+                Template = GetTemplate(),
+            };
+
+            var child = new Control();
+            target.Content = child;
+            target.ApplyTemplate();
+            ((ContentPresenter)target.Presenter).UpdateChild();
+
+            Assert.Equal(child.Parent, target);
+            Assert.Equal(child.GetLogicalParent(), target);
+            Assert.Equal(new[] { child }, target.GetLogicalChildren());
+        }
+
         [Fact]
         public void DataTemplate_Created_Control_Should_Be_Logical_Child_After_ApplyTemplate()
         {