diff --git a/datasette/app.py b/datasette/app.py
index 731c3d13..3adb8925 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -1058,7 +1058,11 @@ class Datasette:
                 filepath = os.path.join(self.plugins_dir, filename)
                 with open(filepath) as f:
                     mod = module_from_path(filepath, name=filename)
-                    pm.register(mod)
+                    try:
+                        pm.register(mod)
+                    except ValueError:
+                        # Plugin already registered
+                        pass
 
     def app_css_hash(self):
         if not hasattr(self, '_app_css_hash'):
diff --git a/tests/fixtures.py b/tests/fixtures.py
index 9889f2b4..38df8b00 100644
--- a/tests/fixtures.py
+++ b/tests/fixtures.py
@@ -15,12 +15,16 @@ def app_client():
         conn = sqlite3.connect(filepath)
         conn.executescript(TABLES)
         os.chdir(os.path.dirname(filepath))
+        plugins_dir = os.path.join(tmpdir, 'plugins')
+        os.mkdir(plugins_dir)
+        open(os.path.join(plugins_dir, 'my_plugin.py'), 'w').write(PLUGIN)
         ds = Datasette(
             [filepath],
             page_size=50,
             max_returned_rows=100,
             sql_time_limit_ms=20,
             metadata=METADATA,
+            plugins_dir=plugins_dir,
         )
         ds.sqlite_functions.append(
             ('sleep', 1, lambda n: time.sleep(float(n))),
@@ -90,6 +94,20 @@ METADATA = {
     }
 }
 
+PLUGIN = '''
+from datasette import hookimpl
+import pint
+
+ureg = pint.UnitRegistry()
+
+
+@hookimpl
+def prepare_connection(conn):
+    def convert_units(amount, from_, to_):
+        "select convert_units(100, 'm', 'ft');"
+        return (amount * ureg(from_)).to(to_).to_tuple()[0]
+    conn.create_function('convert_units', 3, convert_units)
+'''
 
 TABLES = '''
 CREATE TABLE simple_primary_key (
diff --git a/tests/test_api.py b/tests/test_api.py
index 050359d3..21fc34bf 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -588,8 +588,10 @@ def test_row_foreign_key_tables(app_client):
 
 
 def test_unit_filters(app_client):
-    response = app_client.get('/test_tables/units.json?distance__lt=75km&frequency__gt=1kHz',
-                              gather_request=False)
+    response = app_client.get(
+        '/test_tables/units.json?distance__lt=75km&frequency__gt=1kHz',
+        gather_request=False
+    )
     assert response.status == 200
     data = response.json
 
@@ -598,3 +600,11 @@ def test_unit_filters(app_client):
 
     assert len(data['rows']) == 1
     assert data['rows'][0][0] == 2
+
+
+def test_plugins_dir_plugin(app_client):
+    response = app_client.get(
+        "/test_tables.json?sql=select+convert_units(100%2C+'m'%2C+'ft')",
+        gather_request=False
+    )
+    assert pytest.approx(328.0839) == response.json['rows'][0][0]
diff --git a/tests/test_utils.py b/tests/test_utils.py
index d71ecc60..3b3b8946 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -195,6 +195,7 @@ def test_temporary_docker_directory_uses_hard_link():
             extra_options=None,
             branch=None,
             template_dir=None,
+            plugins_dir=None,
             static=[],
         ) as temp_docker:
             hello = os.path.join(temp_docker, 'hello')
@@ -218,6 +219,7 @@ def test_temporary_docker_directory_uses_copy_if_hard_link_fails(mock_link):
             extra_options=None,
             branch=None,
             template_dir=None,
+            plugins_dir=None,
             static=[],
         ) as temp_docker:
             hello = os.path.join(temp_docker, 'hello')