Can anyone explain to me what this compile error means or what could have caused it?

ld: multiple definitions of symbol .objc_class_name_ClassName

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It means the same General/ObjC class is being defined in two .m files. You can’t do that. Perhaps you put @implementation in a header file? – General/MikeTrent

Or loaded the .m file from a header file; use @class in headers wherever possible! – General/RobRix

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Possibly one more reason. If Xcode has more than one reference to some class (h+m file pair), linking will fail but only with “release” build config being active.