Multiple fractions can pull in module.xml files defining the same module. However, we have multiple module-rewrite.conf files scattered around the source tree, and it may happen that some module.xml files are rewritten and some are not, resulting in different definitions of the same module. Which, in the end, means that the classloading environment is nondeterministic, which is wrong.
We should have a test, running at the very end of the build, or at least after all fractions are built, that will scan all the target directories (because in src, the module.xml files haven't been processed by the fraction plugin yet), find all module.xml files, detect duplicate module definitions, and verify they are identical.
If we find different module.xml files defining the same module, we need to fix that.