Android : How to avoid "Multiple dex files define" on IMarketBillingService

I need to include a complex Android Library into my own app, but this library have the IMarketBillingService.aidl and my app also...

So I've this error = Dex Loader] Unable to execute dex: Multiple dex files define Lcom/android/vending/billing/IMarketBillingService;

If I remove this AIDL in my app or in the lib, it doesn't compile anymore (IMarketBillingService object is undefined, so the classes doesn't compile)

How do I do ?

Thanks in advance :)