Android Library integration failed

Hi, I'm trying to build an android library as plugin for unity. Got it already working in a previous version, but now the build fails with this error:

     CommandInvokationFailure: Gradle build failed. 
       Program type already present: com.unity3d.player.Camera2Wrapper

I included the Unity classes.jar into the libs folder of the android library, so i can access main Context and activity. I believe that tis is causing the error.

but i have no Idea how to solve this. Others with this problem, hat more than one version of the same library in their project, but I can't see anything like that in my project. Any help would be apreciated.