New Windows (.xaml files) open twice in VS and intellisesense doesn't fully load til repopen

* .NET Core Version: Version: 3.1.100-preview3-014613
* Windows version: 18363.418
* Does the bug reproduce also in WPF for .NET Framework 4.8?: No
* Is this bug related specifically to tooling in Visual Studio?. No
* Security issues and bugs should be reported privately, learn more via our [responsible disclosure guidelines](https://github.com/dotnet/wpf/blob/master/README.md#reporting-security-issues-and-security-bugs).

**Problem description:**
When creating a new Window (.xaml) file in an active .NET Core wpf app, it creates the file, and opens 2 tabs of it. One contains the normal .xaml and visual designer and one only contains the designer. Additionally, the designer doesn't fully load until you close the tabs and reopen them.
**Actual behavior:**
2 tabs are opened, intellisense for designer doesn't load.

Infinite:

**Expected behavior:**
Only 1 tab is opened for the desginer
**Minimal repro:**
Create a new C# WPF app (.NET core)
Add a new Window (WPF) file to the project