MetroUI:Another user has already installed an unpackaged version of this application.

If you have had to delete your user profile in Windows 8 to get around the known Internet Explorer 10 bug, you might have run into this error when trying to run an app from Visual Studio

Error    2    Another user has already installed an unpackaged version of this application.                    The current user cannot replace this with a packaged version.                    The conflicting package is 00392a51-cf7d-4fd5-bde7-be48dcef9ec2 and it was published by CN=sempf     ToDoTodayApp

The reason this happens is the Package Name parameter of Metro apps.  In the Package.appxmanifest you can see the GIUD that is used for the unique app identity.

<Package xmlns="http://schemas.microsoft.com/appx/2010/manifest">
  <Identity Name="00392a51-cf7d-4fd5-bde7-be48dcef9ec2" Version="1.0.0.0" Publisher="CN=sempf" />
  <Properties>
    <DisplayName>ToDoTodayApp</DisplayName>
    <Description>ToDoTodayApp</Description>
    <PublisherDisplayName>sempf</PublisherDisplayName>
    <Logo>images\storelogo.png</Logo>
  </Properties>

All I had to do to get around the problem was alter the GUID, and the program ran fine.