Unable to convert classes into dex format Unity - google-play

Question

Using the google play games services and Admob plugin I cannot seem to get my back to build into an APK and I am receiving the following error message. Any help would be appreciated as I have removed some duplicate files but I am still failing to find why I cannot build my app.
I am using Unity Unity 5.6.1f1 (64-bit).
game services: https://github.com/playgameservices/play-games-plugin-for-unity Ads: https://github.com/unity-plugins/Firebase-Admob-Unity
CommandInvokationFailure: Unable to convert classes into dex format.
C:/Program Files/Java/jdk1.8.0_121\bin\java.exe -Xmx2048M -Dcom.android.sdkmanager.toolsdir="C:/Users/Jack/AppData/Local/Android/sdk\tools" -Dfile.encoding=UTF8 -jar "C:\Program Files\Unity\Editor\Data\PlaybackEngines\AndroidPlayer/Tools\sdktools.jar" -
stderr[
Uncaught translation error: java.lang.IllegalArgumentException: already added: Lcom/google/android/gms/internal/zzbyb;
Uncaught translation error: java.lang.IllegalArgumentException: already added: Lcom/google/android/gms/internal/zzbyc;
2 errors; aborting
]

Answer 1

Some Background
When Unity builds your project for Android, it invokes several tools from the Android SDK. As part of that process, it converts all of your native (Java) code for Android into a file format called DEX (Dalvik executable).
All of your Android plugins get built up into a single package using that tool. The problems start when a few plugins have the same compiled Java code (classes) in them. This will cause the DEX tool to fail with an error like the one you're seeing:
Uncaught translation error: java.lang.IllegalArgumentException:
already added: Lcom/google/android/gms/internal/zzbyb;
Uncaught translation error: java.lang.IllegalArgumentException: already added: Lcom/google/android/gms/internal/zzbyc;
This means that classes with those names were already included in another library, and so they cannot be added again.
Possible Causes
As explained above, this error occurs when you have duplicate plugins in your project; this can happen in different scenarios, here are some examples:
The same plugin is included more than once, under different folders of the project.
The same plugin is included more than once with different versions.
A plugin contains other dependencies "embedded" inside it, but these dependencies are already included in the project in some form.
How To Fix
You should look up duplicate Android plugins in your project and eliminate them (keep only 1 copy). From the error message you posted, the issue here is related to Google play services libraries. You should look into that (libraries named play-services-xxxx.aar).
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Answer 2

I was facing the same problem after add admob in my project, here are 2 possible solutions:
Go to the file >> build settings and change the option internal to gradle
Open libs folders separately of both Facebook SDK and Google Play services and delete the matching files
It would be something like (support v4)

Answer 3

Google ads 11.2 has errors. It reuses certain jar classes .
On its own, it isnot an issue
Once you add in another google class, ie firebase, your done for.
There is a fix though. Google put out firebase 4.2 and ads 11.4