Handle duplicate contents being returned and dummy them. Refs #310

* Unfortunately too late for v0.30, but at least this will help anyone
  who upgrades v0.29 -> v0.31 or such.

renderdocui/3rdparty/WinFormsUI/Docking/DockPanel.Persistor.cs

@@ -608,7 +608,7 @@ namespace WeifenLuo.WinFormsUI.Docking
                 for (int i = 0; i < contents.Length; i++)
                 {
                     IDockContent content = deserializeContent(contents[i].PersistString);
-                    if (content == null)
+                    if (content == null || dockPanel.Contents.IndexOf(content) >= 0)
                         content = new DummyContent();
                     content.DockHandler.DockPanel = dockPanel;
                     content.DockHandler.AutoHidePortion = contents[i].AutoHidePortion;

renderdocui/Windows/MainWindow.cs

@@ -519,6 +519,15 @@ namespace renderdocui.Windows
                     // file is invalid
                     return false;
                 }
+                catch (Exception)
+                {
+                    MessageBox.Show("Something went seriously wrong trying to load the window layout.\n" +
+                        "Trying to recover now, but you might have to delete the layout file in %APPDATA%/renderdoc.\n",
+                        "Error loading window layout",
+                        MessageBoxButtons.OK, MessageBoxIcon.Error);
+
+                    return false;
+                }
                 return true;
             }