title: in app billing Proguard Duplicate zip entry
body: <p>I have faced a problem, for which I cannot find any solution on a internet. I implemented this library into my project: <a href="https://github.com/anjlab/android-inapp-billing-v3" rel="nofollow noreferrer">https://github.com/anjlab/android-inapp-billing-v3</a></p>
  
  <p>after I implemented it, when I try to generate apk this error appears:</p>
  
  <blockquote>
    <p>Warning:Exception while processing task java.io.IOException: Can't write [C:\Users\tomas\StudioProjects\TalisProject\app\build\intermediates\transforms\proguard\release\jars\3\1f\main.jar] (Can't read [C:\Users\tomas\StudioProjects\TalisProject\app\build\intermediates\classes\release(;;;;;;**.class)] (Duplicate zip entry [com/android/vending/billing/IInAppBillingService$Stub$Proxy.class]))</p>
  </blockquote>
  
  <p>After I added this code into my proguard:</p>
  
  <pre><code>-keep class com.android.vending.billing.**
  -outjars  build\intermediates\transforms\proguard\release\jars\3\1f\main.jar
  -outjars out.jar(**.class)
  -dontwarn com.android.vending.billing.**
  </code></pre>
  
  <blockquote>
    <p>Warning:Exception while processing task java.io.IOException: The output jar [C:\Users\tomas\StudioProjects\TalisProject\app\build\intermediates\transforms\proguard\release\jars\3\1f\main.jar] must have a filter, or all subsequent output jars will be empty.</p>
  </blockquote>
  
  <p>Does anyone know how to solve this problem?</p>