Conflicting rules for producing /.../MyClass.o [obj]
while trying to apply: /.../GenericGCC.qbs:242:1 [cpp, c, objcpp, objc] -> [obj]
was already defined in: /.../GenericGCC.qbs:242:1 [cpp, c, objcpp, objc] -> [obj]

I imagine this is because I have both MyClass.cpp and MyClass.mm. QBS should include the original file extension in the object file's name to prevent such conflicts, i.e. MyClass.cpp -> MyClass.cpp.o, MyClass.mm -> MyClass.mm.o