Use a set for the data check registry to prevent double entries

CHANGELOG.rst

@@ -21,6 +21,7 @@ Fixed
 ~~~~~
 
 * Fix installation documentation (nginx, uWSGI).
+* Use a set for data checks registry to prevent double entries.
 
   Removed
 ~~~~~~~

aleksis/core/apps.py

@@ -74,9 +74,9 @@ class CoreConfig(AppConfig):
         """Get all data checks from all loaded models."""
         from aleksis.core.data_checks import DataCheckRegistry
 
-        data_checks = []
+        data_checks = set()
         for model in apps.get_models():
-            data_checks += getattr(model, "data_checks", [])
+            data_checks.update(getattr(model, "data_checks", []))
         DataCheckRegistry.data_checks = data_checks
 
     def preference_updated(

aleksis/core/data_checks.py

@@ -225,7 +225,7 @@ class DataCheck:
 class DataCheckRegistry:
     """Create central registry for all data checks in AlekSIS."""
 
-    data_checks = []
+    data_checks: set = set()
 
     @classproperty
     def data_checks_by_name(cls):