Android Studio:错误:任务':app:dexDebug'的执行失败

我是第一次使用 Android Studio，在导入项目后出现以下错误(之前是一个 eclipse 项目，我也遇到了问题.)

I'm using Android Studio for the first time and I got the following error after importing the project (previously it was an eclipse project where I had issues too.)

以下是给定的信息:

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

app build.gradle 是这样的:

The app build.gradle is this:

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')
}

项目built.gradle是这样的:

And the project built.gradle is this:

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

allprojects {
    repositories {
        jcenter()
    }
}

您知道问题可能是什么以及如何解决吗?对这个问题的一个很好的解释也很有用，因为我是 Android Studio 的新手.提前致谢:)

Do you have any idea what the problem could be and how can it be solved? A good explanation for the problem would be useful too as I am new to Android Studio. Thanks in advance :)