Full error message in Visual Studio output window:
The item "*.resources" was specified more than once in the "Resources" parameter. Duplicate items are not supported by the "Resources" parameter.
I saw this error on context of custom cultures usage. The compiling machine does not have a project used custom culture registered.
Have a look under %WinDir%/Globalization. There should be special *.nlp files for each used custom culture.
After registering a new custom cultures Visual Studio must be closed and new started.