Build Error with AWS SDK iOS

I downloaded the SDK from the Cognito service, and installed it under this instruction (http://ift.tt/1jMcsAv).

I installed the pod, and added the frameworks to my project. When I tried to run my project, I got an error including ld: 6705 duplicate symbols for architecture x86_64.

For example:

duplicate symbol _OBJC_CLASS_$_AWSEC2ResponseSerializer in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) duplicate symbol _OBJC_METACLASS_$_AWSEC2ResponseSerializer in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) duplicate symbol _OBJC_CLASS_$_AWSEC2RequestRetryHandler in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) duplicate symbol _OBJC_METACLASS_$_AWSEC2RequestRetryHandler in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) duplicate symbol _OBJC_CLASS_$_AWSEC2 in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) duplicate symbol _OBJC_METACLASS_$_AWSEC2 in: /Users/haichen/Library/Developer/Xcode/DerivedData/AWSTest-ceywyjkmugdxssfeodliqhmuehlz/Build/Products/Debug-iphonesimulator/libPods-AWSEC2.a(AWSEC2Service.o) /Users/haichen/Documents/AWSTest/aws-ios-sdk-2.1.1/frameworks/AWSEC2.framework/AWSEC2(AWSEC2Service.o) ld: 6705 duplicate symbols for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

Could anyone help me find out the reason?