Dex Loader Error - Generate Google App Engine Backend

There is an error occurred after generate Google App Engine Backend for an existing android project.

Unable to execute dex: Multiple dex files define Lcom/google/api/client/googleapis/extensions/android/accounts/GoogleAccountManager;

I took me hours to figure out what the problem was.

The problem was my app had already added these google jars for my existing coding and developement

C:\WorkSpace\myapp\libs\google-api-client-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-api-client-android-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-android-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-gson-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-jackson-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-jackson2-1.15.0-rc.jar
C:\WorkSpace\myapp\libs\google-oauth-client-1.15.0-rc.jar

But, the GPE kept adding these jars into my library

C:\WorkSpace\myapp\libs\google-api-client-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-api-client-android-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-android-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-gson-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-jackson2-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-http-client-jdo-1.18.0-rc.jar
C:\WorkSpace\myapp\libs\google-oauth-client-1.18.0-rc.jar

I removed conflicting jars by deleting those jars in version 1.15. Mission accomplished.