Using Vistual Studio 2019 to build.
I’m not exactly sure what command unreal build is actually using to link. However the behavior seems to be that 2 copies of a .cpp file with identical functions will compile fine in DebugGameEditor mode, however when compiling in DebugGame mode it complains about duplicate symbols. However if we remove the extra copy of the file, and try to have the symbols come form only one .cpp file, the linker now CAN’T find the symbols when compiling in DebugGameEditor mode.
I’ve created a small project which shows of the behavior at