Google Cloud Endpoints iOS client library: 'duplicate symbols error'

I have a Google App Engine server which uses Google Cloud Endpoints v1. I generated the iOS client library as per these instructions:

https://cloud.google.com/endpoints/docs/frameworks/legacy/v1/java/calling-from-ios#compiling_the_client_library_generator_and_generating_your_library

However when importing the client library files to Xcode, I get multiple ‘duplicate symbols’ errors when running Build. How can I fix these?