创建项目出现这个问题：clang: error: linker command failed with exit code 1

duplicate symbol _OBJC_CLASS_$_ShoppingCarViewController in:
    /Users/apple/Library/Developer/Xcode/DerivedData/demo1-dfevsgimvmtrbfelwahwyqpkazrs/Build/Intermediates/demo1.build/Debug-iphonesimulator/demo1.build/Objects-normal/x86_64/ShoppingCarViewController-D3FF477D55CDCD4D.o
    /Users/apple/Library/Developer/Xcode/DerivedData/demo1-dfevsgimvmtrbfelwahwyqpkazrs/Build/Intermediates/demo1.build/Debug-iphonesimulator/demo1.build/Objects-normal/x86_64/ShoppingCarViewController-952D7020FD41EC3B.o
duplicate symbol _OBJC_METACLASS_$_ShoppingCarViewController in:
    /Users/apple/Library/Developer/Xcode/DerivedData/demo1-dfevsgimvmtrbfelwahwyqpkazrs/Build/Intermediates/demo1.build/Debug-iphonesimulator/demo1.build/Objects-normal/x86_64/ShoppingCarViewController-D3FF477D55CDCD4D.o
    /Users/apple/Library/Developer/Xcode/DerivedData/demo1-dfevsgimvmtrbfelwahwyqpkazrs/Build/Intermediates/demo1.build/Debug-iphonesimulator/demo1.build/Objects-normal/x86_64/ShoppingCarViewController-952D7020FD41EC3B.o
ld: 2 duplicate symbols for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)