When adding a second ad network into the project and trying to build, I get an error. Does anyone know how to fix it? This is the error:

Error building Player: CommandInvokationFailure: Unable to convert classes into dex format. See the Console for details.
C:\Program Files\Java\jdk1.8.0_40\bin\java.exe -Xmx2048M -Dcom.android.sdkmanager.toolsdir="C:/Program Files (x86)/Android SDK/adt-bundle-windows-x86_64-20140702/sdk	ools" -Dfile.encoding=UTF8 -jar "C:/Users/Shay/Documents/Unity/Editor/Data/BuildTargetTools/AndroidPlayer\sdktools.jar" -

stderr[

UNEXPECTED TOP-LEVEL EXCEPTION:
java.lang.IllegalArgumentException: already added: Lcom/google/android/gms/internal/qn;
	at com.android.dx.dex.file.ClassDefsSection.add(ClassDefsSection.java:122)
	at com.android.dx.dex.file.DexFile.add(DexFile.java:161)
	at com.android.dx.command.dexer.Main.processClass(Main.java:732)
	at com.android.dx.command.dexer.Main.processFileBytes(Main.java:673)
	at com.android.dx.command.dexer.Main.access$300(Main.java:83)
	at com.android.dx.command.dexer.Main$1.processFileBytes(Main.java:602)
	at com.android.dx.cf.direct.ClassPathOpener.processArchive(ClassPathOpener.java:284)
	at com.android.dx.cf.direct.ClassPathOpener.processOne(ClassPathOpener.java:166)
	at com.android.dx.cf.direct.ClassPathOpener.processDirectory(ClassPathOpener.java:229)
	at com.android.dx.cf.direct.ClassPathOpener.processOne(ClassPathOpener.java:158)
	at com.android.dx.cf.direct.ClassPathOpener.process(ClassPathOpener.java:144)
	at com.android.dx.command.dexer.Main.processOne(Main.java:632)
	at com.android.dx.command.dexer.Main.processAllFiles(Main.java:510)
	at com.android.dx.command.dexer.Main.runMonoDex(Main.java:280)
	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)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:497)
	at SDKMain.main(SDKMain.java:130)
1 error; aborting
]

The error sounds pretty clear:

java.lang.IllegalArgumentException: already added: Lcom/google/android/gms/internal/qn;

You likely have two instances of Play Services added.