diff --git a/personal_data/util.py b/personal_data/util.py
index 2185520..e1d1003 100644
--- a/personal_data/util.py
+++ b/personal_data/util.py
@@ -58,6 +58,7 @@ def csv_str_to_value(
         return None
     return s
 
+
 def csv_safe_value(v: object) -> str:
     if isinstance(v, urllib.parse.ParseResult):
         return v.geturl()
@@ -91,6 +92,8 @@ def deduplicate_by_ignoring_certain_fields(
         for idx2, second in enumerate(dicts[idx1 + 1 :], idx1 + 1):
             if equals_without_fields(first, second, deduplicate_ignore_columns):
                 to_remove.add(idx2)
+            del idx2, second
+        del idx1, first
 
     to_remove = sorted(to_remove)
     while to_remove:
@@ -100,10 +103,10 @@ def deduplicate_by_ignoring_certain_fields(
 
 
 def deduplicate_dicts(
-    dicts: Sequence[dict],
+    dicts: Sequence[dict[str,typing.Any] | frozendict[str,typing.Any]],
     deduplicate_mode: data.DeduplicateMode,
     deduplicate_ignore_columns: list[str],
-) -> tuple[Sequence[dict], list[str]]:
+) -> tuple[Sequence[dict[str,typing.Any]], list[str]]:
     assert isinstance(deduplicate_ignore_columns, list), deduplicate_ignore_columns
 
     fieldnames = []
@@ -133,7 +136,7 @@ def deduplicate_dicts(
     return dicts, fieldnames
 
 
-def normalize_dict(d: dict) -> frozendict:
+def normalize_dict(d: dict[str,typing.Any]) -> frozendict[str,typing.Any]:
     return frozendict(
         {k: csv_str_to_value(str(v)) for k, v in d.items() if csv_str_to_value(str(v)) is not None},
     )