reject a table redefined after its parent table header

tests/test_parser.py

@@ -112,3 +112,18 @@ def test_parser_rejects_tab_in_bare_key(content: str) -> None:
     parser = Parser(content)
     with pytest.raises(ParseError):
         parser.parse()
+
+
+@pytest.mark.parametrize(
+    "content",
+    [
+        "[a.b]\n[a]\n[a.b]",
+        "[a.b]\nx = 1\n[a]\n[a.b]\ny = 2",
+        "[a.b.c]\n[a]\n[a.b.c]",
+        "[a.b]\n[a.c]\n[a]\n[a.b]",
+    ],
+)
+def test_parser_rejects_table_redefined_after_parent(content: str) -> None:
+    parser = Parser(content)
+    with pytest.raises(ParseError):
+        parser.parse()

tomlkit/container.py

@@ -339,6 +339,14 @@ def _validate_table_candidate(self, current: Table, candidate: Table) -> None:
                     raise KeyAlreadyPresent(k)
                 if k.is_dotted():
                     raise TOMLKitError("Redefinition of an existing table")
+                if isinstance(existing, Table) and isinstance(v, Table):
+                    if not existing.is_super_table() and not v.is_super_table():
+                        # Both sides are concrete `[table]` definitions of the
+                        # same name; the table is declared twice.
+                        raise KeyAlreadyPresent(k)
+                    # One side is still an implicit/super table, so a duplicate
+                    # (if any) is nested deeper - keep checking the subtree.
+                    self._validate_table_candidate(existing, v)
                 continue
 
             if not k.is_dotted():