In my case, I was linking static libraries into my app, and I had OTHER_LDFLAGS=-all_load in my build settings. This causes Xcode to load all members of any static archive library with an x86_64 architecture. So Xcode erroneously tried to link some macOS static libs into my (Intel) iOS Simulator executable. Removing the -all_load linker flag resolved my issue. See forum post about Linker "duplicate symbol" warnings.