Closes #18191: Remove duplicate SQL indexes (#19074)

* Closes #18191: Remove redundant SQL indexes

* Update developer documentation

* Add a system check for duplicate indexes

netbox/core/checks.py

@@ -0,0 +1,41 @@
+from django.core.checks import Error, register, Tags
+from django.db.models import Index, UniqueConstraint
+from django.apps import apps
+
+__all__ = (
+    'check_duplicate_indexes',
+)
+
+
+@register(Tags.models)
+def check_duplicate_indexes(app_configs, **kwargs):
+    """
+    Check for an index which is redundant to a declared unique constraint.
+    """
+    errors = []
+
+    for model in apps.get_models():
+        if not (meta := getattr(model, "_meta", None)):
+            continue
+
+        index_fields = {
+            tuple(index.fields) for index in getattr(meta, 'indexes', [])
+            if isinstance(index, Index)
+        }
+        constraint_fields = {
+            tuple(constraint.fields) for constraint in getattr(meta, 'constraints', [])
+            if isinstance(constraint, UniqueConstraint)
+        }
+
+        # Find overlapping definitions
+        if duplicated := index_fields & constraint_fields:
+            for fields in duplicated:
+                errors.append(
+                    Error(
+                        f"Model '{model.__name__}' defines the same field set {fields} in both `Meta.indexes` and "
+                        f"`Meta.constraints`.",
+                        obj=model,
+                    )
+                )
+
+    return errors