Android : Android Programming Multiple dex files define

I was having some problem when trying to import zxing-core.jar into my project for QR code scanning purpose. I have some other .jar like achartengine, facebook sdk, google library all those and it was working fine until I added the zxing.jar. I am getting the error message as:

[2015-01-31 19:00:40 - eNeighbourhood] Dx 
trouble writing output: already prepared
[2015-01-31 19:00:41 - Dex Loader] Unable to execute dex: Multiple dex files define Lcom/google/zxing/BarcodeFormat;
[2015-01-31 19:00:41 - eNeighbourhood] Conversion to Dalvik format failed: Unable to execute dex: Multiple dex files define Lcom/google/zxing/BarcodeFormat;

I wonder is there any way to fix this? Because if I used back the same code but for an empty project, the zxing works fine.

Thanks in advance.