One of Guido's mentees (I don't know her bpo account name) also ran into this:

https://python.zulipchat.com/#narrow/stream/116742-core.2Fhelp/topic/Weird.20sphinx.20error

Some analysis shows that we do have a bug in the docs – the sphinx '.. module::' directive is only supposed to be used once per module, because it means "this is the canonical documentation for this module, that should be linked to whenever anyone mentions this module".

But we have two documents that use '.. module:: email.message':
  Doc/library/email.message.rst
  Doc/library/email.compat32-message.rst

I don't know enough about the email module to know what the right fix is, so I'll CC the email-interest-area folks too... maybe email.compat32-message.rst should be using '.. currentmodule:: email.message' instead of '.. module::'?

It's not clear why this error is appearing and disappearing for different people. The sphinx 2.1 release did make some changes in this area:

  https://github.com/sphinx-doc/sphinx/commit/259be8716ad4b2332aa4d7693d73400eb06fa7d7

but that doesn't explain why it appears and disappears randomly.

Probably not worth tracking down though, given that we can just fix the underlying problem.