External lib link error in a Visual Studio project within several *.cu files

After I split kernel.cu into several files, there are some errors like:
error LNK2005: “void (__stdcall* glBindBuffer)(unsigned int,unsigned int)” (?glBindBuffer@@3P6GXII@ZA) already defined in generate.cu.obj.

My project include external lib (openGL, glut). And there is no error when I use single file.
I’m using Visual Studio 2012. What can I do to fix this problem?