Today I encountered a weird problem when trying to use IRremote library, and I managed to strip down the problem as following. If you have a folder in libraries, with Foo.h and Foo.cpp inside, and write a sketch to include Foo.h:

Foo.h

#ifndef Foo_H
#define Foo_H

int AAA() {
    return 0;
}

#endif

Foo.cpp

#include "Foo.h"

Sketch

#include <Foo.h>

void setup(){

}

void loop(){

}

The error message is:

 Foo\Foo.cpp.o: In function `AAA()':

 E:\workShop\Arduino\libraries\Foo\/Foo.h:5: multiple definition of `AAA()'

 includeTest.cpp.o:E:\workShop\Arduino\libraries\Foo/Foo.h:5:

 first defined here

I'm using a Windows 7 32-bit machine. Tested on Arduino 1.0.5, 1.0.4, and 21, 22.

So with some research I figured out the problem comes from my confusion of preprocessor and linking. This question explains how preprocessor includes file and include guard:

These are some of the pages helped me understand linking:

And this is a better explanation of inline specifier:

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