Android 我不能理解这个错误

我有这个错误的问题，当我建立我的项目，它给了我这种错误

Error:Android Dex: [Logintest] Unable to execute DX
Error:Android Dex: [Logintest] com.android.dex.DexException: Multiple dex files define Landroid/support/v4/accessibilityservice/AccessibilityServiceInfoCompat$AccessibilityServiceInfoVersionImpl;
Error:Android Dex: [Logintest] at com.android.dx.merge.DexMerger.readSortableTypes(DexMerger.java:594)
Error:Android Dex: [Logintest] at com.android.dx.merge.DexMerger.getSortedTypes(DexMerger.java:552)
Error:Android Dex: [Logintest] at com.android.dx.merge.DexMerger.mergeClassDefs(DexMerger.java:533)
Error:Android Dex: [Logintest] at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:170)
Error:Android Dex: [Logintest] at com.android.dx.merge.DexMerger.merge(DexMerger.java:188)
Error:Android Dex: [Logintest] at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:439)
Error:Android Dex: [Logintest] at com.android.dx.command.dexer.Main.runMonoDex(Main.java:287)
Error:Android Dex: [Logintest] at com.android.dx.command.dexer.Main.run(Main.java:230)
Error:Android Dex: [Logintest] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
Error:Android Dex: [Logintest] at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
Error:Android Dex: [Logintest] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
Error:Android Dex: [Logintest] at java.lang.reflect.Method.invoke(Method.java:483)
Error:Android Dex: [Logintest] at org.jetbrains.android.compiler.tools.AndroidDxRunner.runDex(AndroidDxRunner.java:161)
Error:Android Dex: [Logintest] at org.jetbrains.android.compiler.tools.AndroidDxRunner.main(AndroidDxRunner.java:294)
Error:Android Dex: [Logintest] at com.intellij.rt.execution.CommandLineWrapper.main(CommandLineWrapper.java:121)

---

尝试清理项目。请参阅“您使用外部库吗？如何清理项目？”。我还是这个Android的新手。我如何知道我正在使用外部库？