error compiling

The error is following. can anyone help me solve it?
I ve already put my DAC_shield folder in my Documents/Arduino/libraries. And all the ports are good. And I can run my code on my friends' computer.

"Arduino: 1.6.7 (Windows 7), Board: "Arduino Due (Programming Port)"

libraries\DAC_shield\DAC_shield.cpp.o: In function `DAC_shield::set(unsigned char, unsigned char)':

C:\Users\y\Documents\Arduino\libraries\DAC_shield/DAC_shield.cpp:23: multiple definition of `DAC_shield::set(unsigned char, unsigned char)'

sketch\DAC_shield.cpp.o:sketch/DAC_shield.cpp:23: first defined here

libraries\DAC_shield\DAC_shield.cpp.o: In function `DAC_shield::prep()':

C:\Users\y\Documents\Arduino\libraries\DAC_shield/DAC_shield.cpp:8: multiple definition of `DAC_shield::prep()'

sketch\DAC_shield.cpp.o:sketch/DAC_shield.cpp:8: first defined here

collect2.exe: error: ld returned 1 exit status

Multiple libraries were found for "DAC_shield.h"
Used: C:\Users\y\Documents\Arduino\libraries\DAC_shield
Not used: C:\Program Files\Arduino\libraries\DAC_shield
exit status 1
Error compiling.
"

Somehow it's trying to compile DAC shield library twice. Do you have the DACshield files also located in the same folder as your sketch in addition to the library folder? Don't do that - if you have the library installed, it won't work if you try to put the library in the same folder as the sketch.

Remove the library from "C:\Program Files\Arduino\libraries\DAC_shield "
In other words, delete the folder C:\Program Files\Arduino\libraries\DAC_shield
OR
Cut it and paste it in a new folder with name "InCaseIMessUp" in my documents, as a backup.