Building the standard assets project fails after building once, due to certificate errors. Removing the WSATestCertificate files and settings allows to build once. After that the same issue reoccurs.

To reproduce:
1. Import the standard assets project
2. Build for Phone 8.1 or WSA 8.1
3. The first build works fine, a new WSATestCertificate is generated
4. Build the same project again
5. Errors are thrown:

!overwrite && (ERROR_FILE_EXISTS == error)
UnityEditor.EditorUtility:WSACreateTestCertificate(String, String, String, Boolean)
PostProcessMetroPlayer:PostProcessSDKSpecific(WSASDK, BuildTarget, BuildOptions, String, String, ProjectImages, LibraryCollection) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:777)
PostProcessMetroPlayer:PostProcess(BuildTarget, BuildOptions, String, String, String, String) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:597)
UnityEditor.HostView:OnGUI()

hr == HRESULT_FROM_WIN32(ERROR_FILE_EXISTS)
UnityEditor.EditorUtility:WSACreateTestCertificate(String, String, String, Boolean)
PostProcessMetroPlayer:PostProcessSDKSpecific(WSASDK, BuildTarget, BuildOptions, String, String, ProjectImages, LibraryCollection) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:777)
PostProcessMetroPlayer:PostProcess(BuildTarget, BuildOptions, String, String, String, String) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:597)
UnityEditor.HostView:OnGUI()

UnityException: Failed to create test certificate.
PostProcessMetroPlayer.PostProcessSDKSpecific (WSASDK targetSDK, BuildTarget target, BuildOptions options, System.String playerPackage, System.String stagingArea, .ProjectImages images, .LibraryCollection libraryCollection) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:777)
PostProcessMetroPlayer.PostProcess (BuildTarget target, BuildOptions options, System.String installPath, System.String stagingAreaData, System.String stagingArea, System.String playerPackage) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/PostProcessMetroPlayer.cs:597)
UnityEditor.Metro.BuildPostprocessor.PostProcess (BuildPostProcessArgs args) (at C:/buildslave/unity/build/PlatformDependent/MetroPlayer/Extensions/Managed/ExtensionModule.cs:84)
UnityEditor.PostprocessBuildPlayer.Postprocess (BuildTarget target, System.String installPath, System.String companyName, System.String productName, Int32 width, Int32 height, System.String downloadWebplayerUrl, System.String manualDownloadWebplayerUrl, BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/PostprocessBuildPlayer.cs:316)
UnityEditor.HostView:OnGUI()