diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 237ea478fac..6163c599614 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -9,6 +9,15 @@
 * When loading plugins, import errors which contain non-ascii messages are now properly handled in Python 2 (`#1998`_).
   Thanks `@nicoddemus`_ for the PR.
 
+* Fixed false-positives warnings from assertion rewrite hook for modules that were rewritten but
+  were later marked explicitly by ``pytest.register_assert_rewrite``
+  or implicitly as a plugin (`#2005`_).
+  Thanks `@RonnyPfannschmidt`_ for the report and `@nicoddemus`_ for the PR.
+
+*
+
+*
+
 *
 
 
@@ -16,7 +25,7 @@
 
 .. _#1976: https://github.com/pytest-dev/pytest/issues/1976
 .. _#1998: https://github.com/pytest-dev/pytest/issues/1998
-
+.. _#2005: https://github.com/pytest-dev/pytest/issues/2005
 
 
 3.0.3
diff --git a/_pytest/assertion/rewrite.py b/_pytest/assertion/rewrite.py
index 80d6ee3ba73..6b4c1f48357 100644
--- a/_pytest/assertion/rewrite.py
+++ b/_pytest/assertion/rewrite.py
@@ -51,6 +51,7 @@ def __init__(self, config):
         self.fnpats = config.getini("python_files")
         self.session = None
         self.modules = {}
+        self._rewritten_names = set()
         self._register_with_pkg_resources()
         self._must_rewrite = set()
 
@@ -92,6 +93,8 @@ def find_module(self, name, path=None):
         if not self._should_rewrite(name, fn_pypath, state):
             return None
 
+        self._rewritten_names.add(name)
+
         # The requested module looks like a test file, so rewrite it. This is
         # the most magical part of the process: load the source, rewrite the
         # asserts, and load the rewritten source. We also cache the rewritten
@@ -178,7 +181,9 @@ def mark_rewrite(self, *names):
         """
         already_imported = set(names).intersection(set(sys.modules))
         if already_imported:
-            self._warn_already_imported(already_imported)
+            for name in names:
+                if name not in self._rewritten_names:
+                    self._warn_already_imported(already_imported)
         self._must_rewrite.update(names)
 
     def _warn_already_imported(self, names):
diff --git a/testing/test_assertrewrite.py b/testing/test_assertrewrite.py
index cedd435f881..eb5070862d1 100644
--- a/testing/test_assertrewrite.py
+++ b/testing/test_assertrewrite.py
@@ -543,6 +543,22 @@ def test_rewritten():
         ''')
         assert testdir.runpytest_subprocess().ret == 0
 
+    def test_remember_rewritten_modules(self, pytestconfig, testdir, monkeypatch):
+        """
+        AssertionRewriteHook should remember rewritten modules so it
+        doesn't give false positives (#2005).
+        """
+        monkeypatch.syspath_prepend(testdir.tmpdir)
+        testdir.makepyfile(test_rewrite_module_warning_once='')
+        warnings = []
+        hook = AssertionRewritingHook(pytestconfig)
+        monkeypatch.setattr(hook.config, 'warn', lambda code, msg: warnings.append(msg))
+        hook.find_module('test_rewrite_module_warning_once')
+        hook.load_module('test_rewrite_module_warning_once')
+        hook.mark_rewrite('test_rewrite_module_warning_once')
+        hook.mark_rewrite('test_rewrite_module_warning_once')
+        assert warnings == []
+
 
 class TestAssertionRewriteHookDetails(object):
     def test_loader_is_package_false_for_module(self, testdir):