The first thing to do is ensure you do a "clean build" that option deletes all the output files before you start and does a full build. Sometimes the problem goes away it is just because adding files had been known to confused it....
sometimes if fails and then you do a normal build and it works the seconds time. This is normally because you have a circular dependence A needs B and B needs A. For the second build B exists from the first so A now works and then B get built again. Another system is the project is always out of date and it thinks it needs to be built.
You need to fix that. Using Submodules is one way.
