I am getting this linker error.
mfcs80.lib(dllmodul.obj) : error LNK2005: _DllMain@12 already defined in MSVCRT.lib(dllmain.obj)
Make sure you include "Stdafx.h" at the top of each .cpp file. I was getting the exact same error and had a single .cpp file that did not include this header at all. Adding the #include solved the problem.