Move handling of duplicate files

This removes the hack added in https://github.com/pytest-dev/pytest/pull/3802.

Adjusts test:

- it appears to not have been changed to 7 intentionally.
- removes XXX comment, likely not relevant anymore since 6dac7743.

testing/test_session.py

@@ -218,7 +218,7 @@ class TestNewSession(SessionTests):
         started = reprec.getcalls("pytest_collectstart")
         finished = reprec.getreports("pytest_collectreport")
         assert len(started) == len(finished)
-        assert len(started) == 7  # XXX extra TopCollector
+        assert len(started) == 8
         colfail = [x for x in finished if x.failed]
         assert len(colfail) == 1