VC出现error LNK2005 错误应该怎么办??

--------------------Configuration: void - Win32 Debug--------------------
Compiling...
file1.cpp
file2.cpp
Linking...
file2.obj : error LNK2005: "double __cdecl add(double,double)" (?add@@YANNN@Z) already defined in file1.obj
Debug oid.exe : fatal error LNK1169: one or more multiply defined symbols found
执行 link.exe 时出错.

void.exe - 1 error(s), 0 warning(s)

--------file1.cpp--

double add(double a, double b)
{
return a + b;
}

-----file2.cpp

#include "file1.cpp"
#include

void main()
{
double a,b;
double e,f;
a = 3;
b = 2;
e = add(a,2);
f = add(b+1,2);
cout<}

只能include头文件，#include "file1.cpp" 改成#include "file1.h"

在abc.h文件中写函数ave的声明，然后在main.cpp中#include "abc.h" 或者 在工程中排除掉abc.cpp文件