Improve numbering numId lookup

src/docx/oxml/numbering.py

@@ -102,7 +102,7 @@ def _next_numId(self):
         """The first ``numId`` unused by a ``<w:num>`` element, starting at 1 and
         filling any gaps in numbering between existing ``<w:num>`` elements."""
         numId_strs = self.xpath("./w:num/@w:numId")
-        num_ids = [int(numId_str) for numId_str in numId_strs]
+        num_ids = {int(numId_str) for numId_str in numId_strs}
         for num in range(1, len(num_ids) + 2):
             if num not in num_ids:
                 break

tests/oxml/test_numbering.py

@@ -0,0 +1,26 @@
+"""Test suite for the docx.oxml.numbering module."""
+
+from typing import cast
+
+import pytest
+
+from docx.oxml.numbering import CT_Numbering
+
+from ..unitutil.cxml import element
+
+
+class DescribeCT_Numbering:
+    @pytest.mark.parametrize(
+        ("num_ids", "expected_value"),
+        [
+            ([], 1),
+            ([1, 2, 3], 4),
+            ([1, 3, 4], 2),
+        ],
+    )
+    def it_knows_the_next_unused_numId(self, num_ids: list[int], expected_value: int):
+        children = ",".join(f"w:num{{w:numId={num_id}}}" for num_id in num_ids)
+        cxml = "w:numbering" if not children else f"w:numbering/({children})"
+        numbering = cast(CT_Numbering, element(cxml))
+
+        assert numbering._next_numId == expected_value