Unable to convert into dex format

Hi, i'm using Google Admob and Open IAB plugin now. Everything seems OK, but when i build to apk, it showed "Unable to convert into dex format..." with many text.
I tried to deleted some jar file in the plugin folder, and only when i deleted unity-plugin-library.jar, i could build the apk. But the problem is advertising didn't show anymore
I tested on Google Admob and Soomla plugin, but the result still the same, so anyone can help me