That doesn't appear to be the case for me, John.
On my end it appears to be an issue with 1.12.2 and older, perhaps a collision between older(i.e. previously-built Forge version jsons) and current(updated by launcher) version formats?
From what I've seen it appears that during the download for a library defined in both parent and child a second copy of the library gets concatenated with the first one
To me this looks like the launcher (or a library it uses) is set up with resumable downloads enabled, but the server isn’t.