“System.Threading.Tasks预定义的类型。任务'在全局别名的多个程序集中定义

I just have set up a new asp.net mvc 4 project, using the latest .net framework (4.5) in combination with Visual studio 2012. After I added some classes, side projects etc. I suddently notice that I'm receiving 5 times the same warning when I compile the project. The warning states:I just have set up a new asp.net mvc 4 project,