When fixing difflib.rst doctests, I used doctest.testfile, which a) runs the tests without any extra sphinx directive and b) reported that there were 47 tests. (I am not sure if that counts 'statements' or 'contiguous statement blocks, but this does not matter here.) DocTestSuite appears to count each docstring as one test. Running with -v:

...  # Normal unittests
Differ (difflib)
Doctest: difflib.Differ ... ok
_fancy_replace (difflib.Differ)
Doctest: difflib.Differ._fancy_replace ... ok
...  # Other docstring tests
unified_diff (difflib)
Doctest: difflib.unified_diff ... ok

DocFileSuite sees the entire file essentially 1 docstring (sort of sensible) and therefore as 1 test (instead of the 47 reported by testfile. At the end of -v output:

f:\dev\3x\Doc/library/difflib.rst
Doctest: difflib.rst ... ok

However, this is enough, I believe, to show that the added tests were run.
Getting 51 instead of 52 tests run on main (without -v) says the same.

I suspect that many of the tests are more or less the same in docstring and doc, but supposedly duplicate copies both need testing.