diff --git a/django/core/checks/registry.py b/django/core/checks/registry.py
index 6e2657b0fb..c580c80483 100644
--- a/django/core/checks/registry.py
+++ b/django/core/checks/registry.py
@@ -21,8 +21,8 @@ class Tags:
 class CheckRegistry:
 
     def __init__(self):
-        self.registered_checks = []
-        self.deployment_checks = []
+        self.registered_checks = set()
+        self.deployment_checks = set()
 
     def register(self, check=None, *tags, **kwargs):
         """
@@ -44,11 +44,8 @@ class CheckRegistry:
 
         def inner(check):
             check.tags = tags
-            if kwargs['deploy']:
-                if check not in self.deployment_checks:
-                    self.deployment_checks.append(check)
-            elif check not in self.registered_checks:
-                self.registered_checks.append(check)
+            checks = self.deployment_checks if kwargs['deploy'] else self.registered_checks
+            checks.add(check)
             return check
 
         if callable(check):
diff --git a/django/test/utils.py b/django/test/utils.py
index e1cca58b27..f86b41ae51 100644
--- a/django/test/utils.py
+++ b/django/test/utils.py
@@ -499,12 +499,12 @@ class override_system_checks(TestContextDecorator):
 
     def enable(self):
         self.old_checks = self.registry.registered_checks
-        self.registry.registered_checks = []
+        self.registry.registered_checks = set()
         for check in self.new_checks:
             self.registry.register(check, *getattr(check, 'tags', ()))
         self.old_deployment_checks = self.registry.deployment_checks
         if self.deployment_checks is not None:
-            self.registry.deployment_checks = []
+            self.registry.deployment_checks = set()
             for check in self.deployment_checks:
                 self.registry.register(check, *getattr(check, 'tags', ()), deploy=True)