Following comes from Senior Support Engineer Deepak Mittal.
Issue description:You may notice an error while building SharePoint workflow project with Team Build 2010.
Detailed error message:Default Configuration and Platform0 error(s), 1 warning(s)
$/ABC/Build/abc.csproj - 0 error(s), 1 warning(s)
C:\Builds\1\ABC\abc.csproj(589): "C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Workflow.Targets" cannot be imported again. It was already imported at "C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v10.0\SharePointTools\Microsoft.VisualStudio.SharePoint.targets (37,3)". This is most likely a build authoring error. This subsequent import will be ignored.
1 projects/solutions compiledNo Test ResultsNo Code Coverage ResultsDebug | Mixed Platforms2 error(s), 4 warning(s)$/ABC/Build/ABC.sln - 2 error(s), 4 warning(s)C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Workflow.Targets (121): Compilation failed. Unable to load one or more of the requested types.Retrieve the LoaderExceptions property for more information.
Resolution:Recently working on a case found an issue while building SharePoint project with Team Build 2010. Error message was as mentioned above. As perthe error message, there are some tasks which are defined in both workflow.targets and Microsoft.VisualStudio.SharePoint.targets and getting called during build. And asthe Microsoft.VisualStudio.SharePoint.targets is called first during build, it was throwing error on Workflow.targets. However, it was tough to found from where the Workflow.targets is getting called. We finally found that it was called from inside project's .csproj file. To resolve the issue, open the .csproj file in notepad and comment out the section where workflow.targets file is called.
Thank you very much for this article.
I've had this problem after I installed VS 2012. I removed the section where the workflow.targets file is called and it works!