diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 0b5e7d47829..e2f8c6c48ed 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -385,6 +385,7 @@ class Field(object):
         """
         if not prepared:
             value = self.get_prep_lookup(lookup_type, value)
+            prepared = True
         if hasattr(value, 'get_compiler'):
             value = value.get_compiler(connection=connection)
         if hasattr(value, 'as_sql') or hasattr(value, '_as_sql'):
diff --git a/tests/model_fields/tests.py b/tests/model_fields/tests.py
index 1fb50376256..432984f6bbd 100644
--- a/tests/model_fields/tests.py
+++ b/tests/model_fields/tests.py
@@ -494,3 +494,21 @@ class GenericIPAddressFieldTests(test.TestCase):
         model_field = models.GenericIPAddressField(protocol='IPv6')
         form_field = model_field.formfield()
         self.assertRaises(ValidationError, form_field.clean, '127.0.0.1')
+
+
+class CustomFieldTests(unittest.TestCase):
+
+    def test_14786(self):
+        """
+        Regression test for #14786 -- Test that field values are not prepared
+        twice in get_db_prep_lookup().
+        """
+        prepare_count = [0]
+        class NoopField(models.TextField):
+            def get_prep_value(self, value):
+                prepare_count[0] += 1
+                return super(NoopField, self).get_prep_value(value)
+
+        field = NoopField()
+        field.get_db_prep_lookup('exact', 'TEST', connection=connection, prepared=False)
+        self.assertEqual(prepare_count[0], 1)