Error 255 The "GenerateWinPRTManifestV2" task failed unexpectedly.
System.ArgumentException: An item with the same key has already been added.
at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
at Microsoft.Silverlight.Build.Tasks.GenerateWinPRTManifestV2.CCIHarvestRegistrationInformation(ProcessWinmd processWinmd, Dictionary`2 inprocServers)
It turns out that there were two projects referencing different copies of the same library and MSBuild wasn't able to differentiate between them. (They weren't strongly named.)
The fix was just to make sure that all the projects in the solution only use one version of any library.
I hope this helps save someone else some time if they hit the issue and end up searching for the same task name.
Remember. Strong names FTW!