Cristina and I have both seen G__std_unordered_map.cxx being created twice by the same build. With parallel builds this manifests itself (except for the output by CMake, stating that it's building this twice) in the second rootcling not finding its temporary file back because the first rootcling to finish has already renamed it.
Cristina and I (with the help of Benedik) have spent about an hour debugging this; we see that the G_std_unordered_map target exists once as core/metautils/G_std_unordered_map and once as core/metautils/unordered_mapDict, and my guess is that it's caused by a missing dependency between them.
I tried to fix the dependency declared in cmake/modules/RootNewMacros.cmake (see attachment) but apparently failed.
Could you have a look, Pere?