"Unable to convert classes into dex format" after update to Unity 5

Today I updated my project to Unity 5. Everything was working fine at first, but as soon as I tried to build my game for android I got an error:

 CommandInvokationFailure: Unable to convert classes into dex format. See the Console for details.
 C:\Program Files\Java\jdk1.8.0_20\bin\java.exe -Xmx2048M - Dcom.android.sdkmanager.toolsdir="C:/Program Files/Android/android-sdk\tools" -Dfile.encoding=UTF8 -jar "C:/Program Files/Unity/Editor/Data/BuildTargetTools/AndroidPlayer\sdktools.jar"

So I took a look at the console output and I found the following lines:

      UNEXPECTED TOP-LEVEL EXCEPTION:
      java.lang.IllegalArgumentException:
  already added:
  Lcom/android/vending/billing/IInAppBillingService$Stub$Proxy;
          at com.android.dx.dex.file.ClassDefsSection.add(ClassDefsSection.java:122)
          at com.android.dx.dex.file.DexFile.add(DexFile.java:161)
          at com.android.dx.command.dexer.Main.processClass(Main.java:732)
          at com.android.dx.command.dexer.Main.processFileBytes(Main.java:673)
          at com.android.dx.command.dexer.Main.access$300(Main.java:83)
          at com.android.dx.command.dexer.Main$1.processFileBytes(Main.java:602)
          at com.android.dx.cf.direct.ClassPathOpener.processArchive(ClassPathOpener.java:284)
          at com.android.dx.cf.direct.ClassPathOpener.processOne(ClassPathOpener.java:166)
          at com.android.dx.cf.direct.ClassPathOpener.processDirectory(ClassPathOpener.java:229)
          at com.android.dx.cf.direct.ClassPathOpener.processOne(ClassPathOpener.java:158)
          at com.android.dx.cf.direct.ClassPathOpener.process(ClassPathOpener.java:144)
          at com.android.dx.command.dexer.Main.processOne(Main.java:632)
          at com.android.dx.command.dexer.Main.processAllFiles(Main.java:510)
          at com.android.dx.command.dexer.Main.runMonoDex(Main.java:280)
          at com.android.dx.command.dexer.Main.run(Main.java:246)
          at com.android.dx.command.dexer.Main.main(Main.java:215)
          at com.android.dx.command.Main.main(Main.java:106)
          at sun.reflect.NativeMethodAccessorImpl.invoke0(Native
  Method)
          at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
          at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
          at java.lang.reflect.Method.invoke(Method.java:483)
          at SDKMain.main(SDKMain.java:129)

I'm using the following plugins:

• Soomla Store - In App Purchase

• Google Play Games plugin

• Google AdMob plugin

I'm guessing the Soomla Plugin is causing the error, but I can't seem to find a solution. Had someone a similar problem or does anyone know how to solve this issue?

Thanks!