Fix false-positive assert rewrite warnings when using 'pytest_plugins'

pytest would emit false positive warnings about assertion-rewrite when a
module appears multiple times in plugins which depend
on other plugins using the 'pytest_plugins' mechanism

CHANGELOG.rst

@@ -22,6 +22,10 @@
 * Fix error message using ``approx`` with complex numbers (`#2082`_).
   Thanks `@adler-j`_ for the report and `@nicoddemus`_ for the PR.
 
+* Fixed false-positives warnings from assertion rewrite hook for modules imported more than
+  once by the ``pytest_plugins`` mechanism.
+  Thanks `@nicoddemus`_ for the PR.
+
 * Remove internal code meant to support earlier Python 3 versions that produced the side effect
   of leaving ``None`` in ``sys.modules`` when expressions were evaluated by pytest (for example passing a condition
   as a string to ``pytest.mark.skipif``)(`#2103`_).

_pytest/assertion/rewrite.py

@@ -186,16 +186,15 @@ class AssertionRewritingHook(object):
         """
         already_imported = set(names).intersection(set(sys.modules))
         if already_imported:
-            for name in names:
+            for name in already_imported:
                 if name not in self._rewritten_names:
-                    self._warn_already_imported(already_imported)
+                    self._warn_already_imported(name)
         self._must_rewrite.update(names)
 
-    def _warn_already_imported(self, names):
+    def _warn_already_imported(self, name):
         self.config.warn(
             'P1',
-            'Modules are already imported so can not be re-written: %s' %
+            'Module already imported so can not be re-written: %s' % name)
-            ','.join(names))
 
     def load_module(self, name):
         # If there is an existing module object named 'fullname' in

testing/test_assertrewrite.py

@@ -682,6 +682,19 @@ def test_rewritten():
         hook.mark_rewrite('test_remember_rewritten_modules')
         assert warnings == []
 
+    def test_rewrite_warning_using_pytest_plugins(self, testdir, monkeypatch):
+        testdir.makepyfile(**{
+            'conftest.py': "pytest_plugins = ['core', 'gui', 'sci']",
+            'core.py': "",
+            'gui.py': "pytest_plugins = ['core', 'sci']",
+            'sci.py': "pytest_plugins = ['core']",
+            'test_rewrite_warning_pytest_plugins.py': "def test(): pass",
+        })
+        testdir.chdir()
+        result = testdir.runpytest_subprocess()
+        result.stdout.fnmatch_lines(['*= 1 passed in *=*'])
+        assert 'pytest-warning summary' not in result.stdout.str()
+
 
 class TestAssertionRewriteHookDetails(object):
     def test_loader_is_package_false_for_module(self, testdir):