After a rebuild I get :

```
Error:Execution failed for task ':demo:transformDexArchiveWithExternalLibsDexMergerForMockDebug'.
> com.android.builder.dexing.DexArchiveMergerException: com.android.tools.r8.errors.CompilationError: Program type already present: com.google.android.gms.common.internal.zzag

```

What does that error message mean? What is wrong?