diff --git a/CHANGES.rst b/CHANGES.rst
index 8e9963bba..3ce17d0f3 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -24,7 +24,12 @@ want to know what's different in 5.0 since 4.5.x, see :ref:`whatsnew5x`.
 Unreleased
 ----------
 
-- Nothing yet.
+- Plugins (like the `Django coverage plugin`_) were generating "Already
+  imported a file that will be measured" warnings about Django itself.  These
+  have been fixed, closing `issue 1150`_.
+
+.. _Django coverage plugin: https://pypi.org/project/django-coverage-plugin/
+.. _issue 1150: https://github.com/nedbat/coveragepy/issues/1150
 
 
 .. _changes_56b1:
diff --git a/coverage/inorout.py b/coverage/inorout.py
index b023db0b3..532634ebe 100644
--- a/coverage/inorout.py
+++ b/coverage/inorout.py
@@ -481,6 +481,11 @@ def warn_already_imported_files(self):
                     continue
 
                 disp = self.should_trace(filename)
+                if disp.has_dynamic_filename:
+                    # A plugin with dynamic filenames: the Python file
+                    # shouldn't cause a warning, since it won't be the subject
+                    # of tracing anyway.
+                    continue
                 if disp.trace:
                     msg = "Already imported a file that will be measured: {}".format(filename)
                     self.warn(msg, slug="already-imported")
diff --git a/tests/plugin2.py b/tests/plugin2.py
index c334628ad..60d16206b 100644
--- a/tests/plugin2.py
+++ b/tests/plugin2.py
@@ -7,6 +7,14 @@
 
 import coverage
 
+try:
+    import third.render                 # pylint: disable=unused-import
+except ImportError:
+    # This plugin is used in a few tests. One of them has the third.render
+    # module, but most don't. We need to import it but not use it, so just
+    # try importing it and it's OK if the module doesn't exist.
+    pass
+
 
 class Plugin(coverage.CoveragePlugin):
     """A file tracer plugin for testing."""
diff --git a/tests/test_process.py b/tests/test_process.py
index 2447cffe3..9b451228f 100644
--- a/tests/test_process.py
+++ b/tests/test_process.py
@@ -1673,12 +1673,21 @@ def venv_world_fixture(tmp_path_factory):
         # Create a virtualenv.
         run_command("python -m virtualenv venv")
 
-        # A third-party package that installs two different packages.
+        # A third-party package that installs a few different packages.
         make_file("third_pkg/third/__init__.py", """\
             import fourth
             def third(x):
                 return 3 * x
             """)
+        # Use plugin2.py as third.plugin
+        with open(os.path.join(os.path.dirname(__file__), "plugin2.py")) as f:
+            make_file("third_pkg/third/plugin.py", f.read())
+        # A render function for plugin2 to use for dynamic file names.
+        make_file("third_pkg/third/render.py", """\
+            def render(filename, linenum):
+                return "HTML: {}@{}".format(filename, linenum)
+            """)
+        # Another package that third can use.
         make_file("third_pkg/fourth/__init__.py", """\
             def fourth(x):
                 return 4 * x
@@ -1805,3 +1814,20 @@ def test_venv_isnt_measured(self, coverage_command):
         assert "third" not in out
         assert "coverage" not in out
         assert "colorsys" not in out
+
+    @pytest.mark.skipif(not env.C_TRACER, reason="Plugins are only supported with the C tracer.")
+    def test_venv_with_dynamic_plugin(self, coverage_command):
+        # https://github.com/nedbat/coveragepy/issues/1150
+        # Django coverage plugin was incorrectly getting warnings:
+        # "Already imported: ... django/template/blah.py"
+        # It happened because coverage imported the plugin, which imported
+        # Django, and then the Django files were reported as traceable.
+        self.make_file(".coveragerc", "[run]\nplugins=third.plugin\n")
+        self.make_file("myrender.py", """\
+            import third.render
+            print(third.render.render("hello.html", 1723))
+            """)
+        out = run_in_venv(coverage_command + " run --source=. myrender.py")
+        # The output should not have this warning:
+        # Already imported a file that will be measured: ...third/render.py (already-imported)
+        assert out == "HTML: hello.html@1723\n"