Dev will not let me compile function prototype!!

Here is the prototype:
LRESULT CALLBACK WndProc1(HWND, UINT, WPARAM, LPARAM);

Next I have a class (my own window, called Form).
Then I have the actual WndProc1 CALLBACK.
and that is it. No Main or anything.

It is not known if I leave out the prototype.

Error when using the prototype:
main.cpp D:\Files\C__PRO~1\Game\Dev\main.o(.text+0x38)
multiple definition of `_Z8WndProc1P6HWND__jjl@16'

Does anyone know why Dev did not recognize the function prototype until I renamed the file to a .h file instead of a .cpp file?

I'm not sure why either, but I can offer my 2 cents' explanation.:D

I think you are not supposed to include *.cpp, only *.h .:p

Yes, that was it. Thank you.