Não faço ideia por que esse erro existe:

Program type already present: org.hamcrest.CoreMatchers
Message{kind=ERROR, text=Program type already present: org.hamcrest.CoreMatchers, sources=[Unknown source file], tool name=Optional.of(D8)}

Meu código está no escopo se a dependência de build.gradle (Module: app) for:

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
testImplementation ('junit:junit:4.12'){
    exclude group: 'org.hamcrest', module: 'hamcrest-core'
}
androidTestImplementation 'com.android.support.test:runner:1.0.1'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
implementation 'com.monkeylearn:monkeylearn-java:0.1.4'

}

Atividade principal:

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import com.monkeylearn.MonkeyLearn;
import com.monkeylearn.MonkeyLearnException;
import com.monkeylearn.MonkeyLearnResponse;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        try {
            MonkeyLearn ml = new MonkeyLearn("*******************************");
            String moduleId = "*********";
            String[] textList = {"This is a text to test your classifier", "This is some more text"};
            MonkeyLearnResponse res = ml.classifiers.classify(moduleId, textList, true);
            System.out.println( res.arrayResult );
        } catch (MonkeyLearnException e) {
            e.printStackTrace();
        }
    }
}

Você tem alguma ideia?