diff --git a/tools/import-normalizer/persons_tree.py b/tools/import-normalizer/persons_tree.py
index 66e1b660..af3ceb09 100644
--- a/tools/import-normalizer/persons_tree.py
+++ b/tools/import-normalizer/persons_tree.py
@@ -173,3 +173,37 @@ def _parse_row(row_num: int, fields: dict) -> dict:
         "_spouse_raw": spouse,
         "_bemerkung_raw": bemerkung_out,
     }
+
+
+def _deduplicate(persons: list[dict]) -> tuple[list[dict], list[str]]:
+    """Remove duplicate rows. Two-stage:
+
+    1. Exact (firstName, lastName, birthYear) match.
+    2. (firstName, lastName) where the later entry has birthYear=None and an earlier
+       entry already has a known birthYear.
+    """
+    seen_full: dict[tuple, str] = {}   # (first, last, year) -> rowId
+    seen_name: dict[tuple, str] = {}   # (first, last) -> rowId of first entry with a year
+    result: list[dict] = []
+    skipped: list[str] = []
+
+    for p in persons:
+        first, last, year = p["firstName"], p["lastName"], p["birthYear"]
+        key_full = (first, last, year)
+        key_name = (first, last)
+
+        if key_full in seen_full:
+            skipped.append(f"{p['rowId']} duplicates {seen_full[key_full]} ({first} {last}, year={year})")
+            continue
+
+        if year is None and key_name in seen_name:
+            skipped.append(f"{p['rowId']} duplicates {seen_name[key_name]} ({first} {last}, no birth year)")
+            continue
+
+        seen_full[key_full] = p["rowId"]
+        if year is not None:
+            seen_name[key_name] = p["rowId"]
+
+        result.append(p)
+
+    return result, skipped
diff --git a/tools/import-normalizer/tests/test_persons_tree.py b/tools/import-normalizer/tests/test_persons_tree.py
index 4b509156..ea4a1b61 100644
--- a/tools/import-normalizer/tests/test_persons_tree.py
+++ b/tools/import-normalizer/tests/test_persons_tree.py
@@ -229,3 +229,47 @@ def test_parse_row_empty_spouse_and_notes():
     p = persons_tree._parse_row(4, fields)
     assert p["_spouse_raw"] is None
     assert p["_bemerkung_raw"] is None
+
+
+def test_deduplicate_no_duplicates():
+    persons = [
+        {"rowId": "row_002", "firstName": "Elsgard", "lastName": "Allemeyer", "birthYear": 1920},
+        {"rowId": "row_003", "firstName": "Werner", "lastName": "Allemeyer", "birthYear": 1923},
+    ]
+    result, skipped = persons_tree._deduplicate(persons)
+    assert len(result) == 2
+    assert skipped == []
+
+
+def test_deduplicate_exact_match():
+    # rows 127/138: same firstName, lastName, birthYear
+    persons = [
+        {"rowId": "row_127", "firstName": "Christa", "lastName": "Schütz", "birthYear": 1951},
+        {"rowId": "row_138", "firstName": "Christa", "lastName": "Schütz", "birthYear": 1951},
+    ]
+    result, skipped = persons_tree._deduplicate(persons)
+    assert [p["rowId"] for p in result] == ["row_127"]
+    assert len(skipped) == 1
+    assert "row_138" in skipped[0]
+
+
+def test_deduplicate_none_birth_year_after_known():
+    # rows 129/139: row 129 has birthYear=1964, row 139 has birthYear=None
+    persons = [
+        {"rowId": "row_129", "firstName": "Christoph", "lastName": "Seils", "birthYear": 1964},
+        {"rowId": "row_139", "firstName": "Christoph", "lastName": "Seils", "birthYear": None},
+    ]
+    result, skipped = persons_tree._deduplicate(persons)
+    assert [p["rowId"] for p in result] == ["row_129"]
+    assert len(skipped) == 1
+
+
+def test_deduplicate_both_none_birth_year_kept():
+    # Two people with no birth year but same name: keep first only
+    persons = [
+        {"rowId": "row_A", "firstName": "Hans", "lastName": "Cram", "birthYear": None},
+        {"rowId": "row_B", "firstName": "Hans", "lastName": "Cram", "birthYear": None},
+    ]
+    result, skipped = persons_tree._deduplicate(persons)
+    assert [p["rowId"] for p in result] == ["row_A"]
+    assert len(skipped) == 1