Several errors on new persistent classes

As I was creating a new non-document CoreData app using XCode template, I added two persistent classes, FileServers and Server. I did so by coping it from another app, on which I previously created. Now, I get several errors, each representing an issue own its own (that I cannot fix):
duplicate symbol ‘OBJC_CLASS$_FileServers’ in:
/Users/gordonapple/Library/Developer/Xcode/DerivedData/SimpleSFTPClient-bknoaunyaxmtbnatoytyifidspjy/Build/Intermediates.noindex/SimpleSFTPClient.build/Debug/SimpleSFTPClient.build/Objects-normal/x86_64/FileServers.o
/Users/gordonapple/Library/Developer/Xcode/DerivedData/SimpleSFTPClient-bknoaunyaxmtbnatoytyifidspjy/Build/Intermediates.noindex/SimpleSFTPClient.build/Debug/SimpleSFTPClient.build/Objects-normal/x86_64/FileServers+CoreDataClass.o
So, what am I doing here wrong?