Estou usando o Android Studio pela primeira vez e recebi o seguinte erro depois de importar o projeto (anteriormente era um projeto eclipse em que eu também tinha problemas).

Aqui está a informação fornecida:

Error:Execution failed for task ':app:dexDebug'.
> com.android.ide.common.internal.LoggedErrorException: Failed to run command:
    /home/crash-id/Development/SDK/adt-bundle-linux-x86_64-20140702/sdk/build-tools/21.1.2/dx --dex --no-optimize --output /home/crash-id/AndroidstudioProjects/LocalSin/app/build/intermediates/dex/debug --input-list=/home/crash-id/AndroidstudioProjects/LocalSin/app/build/intermediates/tmp/dex/debug/inputList.txt
  Error Code:
    2
  Output:
    UNEXPECTED TOP-LEVEL EXCEPTION:
    com.android.dex.DexException: Multiple dex files define Lcom/google/ads/AdRequest$ErrorCode;
        at com.android.dx.merge.DexMerger.readSortableTypes(DexMerger.java:596)
        at com.android.dx.merge.DexMerger.getSortedTypes(DexMerger.java:554)
        at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:535)
        at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:171)
        at com.android.dx.merge.DexMerger.merge(DexMerger.java:189)
        at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:454)
        at com.android.dx.command.dexer.Main.runMonoDex(Main.java:303)
        at com.android.dx.command.dexer.Main.run(Main.java:246)
        at com.android.dx.command.dexer.Main.main(Main.java:215)
        at com.android.dx.command.Main.main(Main.java:106)

:app:dexDebug
UNEXPECTED TOP-LEVEL EXCEPTION:
com.android.dex.DexException: Multiple dex files define Lcom/google/ads/AdRequest$ErrorCode;
    at com.android.dx.merge.DexMerger.readSortableTypes(DexMerger.java:596)
    at com.android.dx.merge.DexMerger.getSortedTypes(DexMerger.java:554)
    at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:535)
    at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:171)
    at com.android.dx.merge.DexMerger.merge(DexMerger.java:189)
    at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:454)
    at com.android.dx.command.dexer.Main.runMonoDex(Main.java:303)
    at com.android.dx.command.dexer.Main.run(Main.java:246)
    at com.android.dx.command.dexer.Main.main(Main.java:215)
    at com.android.dx.command.Main.main(Main.java:106)


 FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:dexDebug'.
> com.android.ide.common.internal.LoggedErrorException: Failed to run command:
    /home/crash-id/Development/SDK/adt-bundle-linux-x86_64-20140702/sdk/build-tools/21.1.2/dx --dex --no-optimize --output /home/crash-id/AndroidstudioProjects/LocalSin/app/build/intermediates/dex/debug --input-list=/home/crash-id/AndroidstudioProjects/LocalSin/app/build/intermediates/tmp/dex/debug/inputList.txt
  Error Code:
    2
  Output:

    UNEXPECTED TOP-LEVEL EXCEPTION:
    com.android.dex.DexException: Multiple dex files define Lcom/google/ads/AdRequest$ErrorCode;
        at com.android.dx.merge.DexMerger.readSortableTypes(DexMerger.java:596)
        at com.android.dx.merge.DexMerger.getSortedTypes(DexMerger.java:554)
        at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:535)
        at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:171)
        at com.android.dx.merge.DexMerger.merge(DexMerger.java:189)
        at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:454)
        at com.android.dx.command.dexer.Main.runMonoDex(Main.java:303)
        at com.android.dx.command.dexer.Main.run(Main.java:246)
        at com.android.dx.command.dexer.Main.main(Main.java:215)
        at com.android.dx.command.Main.main(Main.java:106)



* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED

O aplicativo build.gradle é este:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.2"

    defaultConfig {
        applicationId "com.myapp.test"
        minSdkVersion 11
        targetSdkVersion 14
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
        }
    }
}

dependencies {
    compile 'com.android.support:support-v4:18.0.0'
    compile 'com.android.support:appcompat-v7:21.0.3'
    compile 'com.google.android.gms:play-services:+'
    compile files('libs/google-play-services.jar')
    compile files('libs/httpcore-4.3.2.jar')
    compile files('libs/httpmime-4.3.4.jar')
}

E o projeto built.gradle é o seguinte:

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:1.0.0'
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

Você tem alguma idéia de qual poderia ser o problema e como ele pode ser resolvido? Uma boa explicação para o problema também seria útil, pois sou novo no Android Studio. Desde já, obrigado :)