diff --git a/typeguard/pytest_plugin.py b/typeguard/pytest_plugin.py
index b9b5b5f4..caa128b5 100644
--- a/typeguard/pytest_plugin.py
+++ b/typeguard/pytest_plugin.py
@@ -1,3 +1,5 @@
+import sys
+
 from typeguard.importhook import install_import_hook
 
 
@@ -8,8 +10,21 @@ def pytest_addoption(parser):
                          'type checking')
 
 
-def pytest_sessionstart(session):
-    packages = session.config.getoption('typeguard_packages')
-    if packages:
-        package_list = [pkg.strip() for pkg in packages.split(',')]
-        install_import_hook(packages=package_list)
+def pytest_configure(config):
+    value = config.getoption("typeguard_packages")
+    if not value:
+        return
+
+    packages = [pkg.strip() for pkg in value.split(",")]
+
+    already_imported_packages = sorted(
+        package for package in packages if package in sys.modules
+    )
+    if already_imported_packages:
+        message = (
+            "typeguard cannot check these packages because they "
+            "are already imported: {}"
+        )
+        raise RuntimeError(message.format(", ".join(already_imported_packages)))
+
+    install_import_hook(packages=packages)