80080204: App manifest validation error during building XamlIsland apps

[michalleptuch]

Describe the bug

I can't build my XamlIsland apps in Visual Studio 2019 v16.10 and higher. Even Visual Studio 2022 Preview doesn't handle it. Visual Studio 2019 v16.9.8 is the last tested and working version.

Error	APPX0501	Validation error. error 80080204: App manifest validation error: Line 66, Column 27, Reason: The field "*[local-name()='Extensions']/*[local-name()='Extension']/*[local-name()='InProcessServer']/*[local-name()='ActivatableClass']" with value "Microsoft.Toolkit.Win32.UI.XamlHost.XamlApplication" must only be declared once. A duplicate exists on Line 60, Column 27.

Steps to Reproduce

Steps to reproduce the behavior:

1. Clone this repo: https://github.com/michalleptuch/rounded-flyout
2. Open in VS and try to build

Expected behavior

Correctly building of solution

Environment

Project .NET Version:
-  .NET Core 3.1

Packages:
- Microsoft.Toolkit.Win32.UI.XamlApplication (6.0.0-6.1.3)

Windows 10 Build Number:
- 18362-22000

App min and target version:
- 18362-22000

Device form factor:
- Desktop

Visual Studio 
- 2019 (version: 16.10.x) 
- 2022 Preview (version: 17.0)
- 2022

Additional context

I found similar issues that describe the same situations but solutions don't work for me:

• https://stackoverflow.com/questions/64791579/app-manifest-validation-error-after-update-to-visual-studio-2019-16-8-0
• https://developercommunity.visualstudio.com/t/Error-when-building-UWP-app-with-packagi/1258250#T-ND1273557

Hello michalleptuch, thank you for opening an issue with us!

I have automatically added a "needs triage" label to help get things started. Our team will analyze and investigate the issue, and escalate it to the relevant team if possible. Other community members may also look into the issue and provide feedback 🙌

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.

This issue has been marked as "needs attention 👋" due to no activity for 15 days. Please triage the issue so the fix can be established.

[michael-hawker]

FYI @marb2000 @ocalvo

[Samuel12321]

The ModernFlyouts Team is also experiencing this issue.
This is a major breaking bug that has had no solution for 4 months.
@marb2000 @ocalvo @michael-hawker

[atiyka]

I also experienced the same issue when I updated the VisualStudio2019. As a temporal solution I had to go back to the previous version and I can't upgrade until this bug won't be fixed.

[michael-hawker]

Has anyone filed a new issue with the Visual Studio team for this regression? If so, can they link to that issue here?