Adcolony using Admob mediation not working in Flutter app

I am currently using Google Admob to display ads in my app, I want to add mediation as I am unhappy with my fill rate. I followed the steps to integrate Adcolony through mediation by following their steps on the website. I added the adcolony.jar file and synced it with Gradle using Android Studio. During the process of integration I added these lines in my app level build.gradle,

repositories {
    jcenter()

    maven {
      url  "https://adcolony.bintray.com/AdColony"
    }
}

...
dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'androidx.appcompat:appcompat:1.0.2'
    implementation 'com.google.android.gms:play-services-ads:19.0.0'
    implementation 'com.google.ads.mediation:adcolony:4.1.3.1'
}
...

After I run my app, the debug console throws these errors:

Duplicate class com.iab.omid.library.adcolony.walking.a.e found in modules jetified-adcolony.jar (adcolony.jar) and jetified-sdk-4.1.3-runtime.jar (com.adcolony:sdk:4.1.3)
  Duplicate class com.iab.omid.library.adcolony.walking.a.f found in modules jetified-adcolony.jar (adcolony.jar) and jetified-sdk-4.1.3-runtime.jar (com.adcolony:sdk:4.1.3)
  Duplicate class com.iab.omid.library.adcolony.walking.b found in modules jetified-adcolony.jar (adcolony.jar) and jetified-sdk-4.1.3-runtime.jar (com.adcolony:sdk:4.1.3)
  Duplicate class com.iab.omid.library.adcolony.walking.c found in modules jetified-adcolony.jar (adcolony.jar) and jetified-sdk-4.1.3-runtime.jar (com.adcolony:sdk:4.1.3)

  Go to the documentation to learn how to <a href="d.android.com/r/tools/classpath-sync-errors">Fix dependency resolution errors</a>.


* Try:
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BUILD FAILED in 51s
Gradle task assembleDebug failed with exit code 1

I have shortened my errors, but mainly they are to do with duplicate classes found, I don't know how to exclude so many classes in my build.gradle. Please help.

To integrate ad mediation, you need follow all the steps mentioned on the website but rather than using build.gradle I manually added the adapter from the adcolony repo. The Google Admob Mediation website is outdated, use this link to see how to integrate the ad adapter.

https://github.com/AdColony/AdColony-Android-SDK/wiki/Project-Setup