fix: don't reject an out-of-order super-table extended by a dotted key

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Change Log
 
+## [Unreleased]
+
+### Fixed
+
+- Fix valid TOML being rejected when an out-of-order table reopens an implicit super-table with a dotted key: `[a.b.c]` then `[a]` with `b.z = 1` extends the implicit `a.b` super-table rather than redefining it (stdlib `tomllib` accepts it), so it now parses and round-trips instead of raising `Redefinition of an existing table`. The concrete-table case (`[a.b]` then `[a]` `b.z = 1`) stays rejected. ([#565](https://github.com/python-poetry/tomlkit/pull/565))
+
 ## [0.15.1] - 2026-07-17
 
 ### Changed

tests/test_toml_document.py

@@ -643,6 +643,17 @@ def test_valid_out_of_order_independent_tables() -> None:
     assert doc.as_string() == "[a]\nx=1\n[zz]\n[a.b]\nc=1\n"
 
 
+def test_valid_out_of_order_super_table_extended_by_dotted_key() -> None:
+    # A deep header [a.b.c] makes a.b an implicit super-table; reopening [a] and
+    # extending a.b via a dotted key (b.z) only adds a key to that super-table,
+    # it is not a table redefinition.  stdlib tomllib accepts this, so it must
+    # parse and round-trip byte-for-byte rather than raise. (The concrete-table
+    # form `[a.b]` then `[a]` `b.z=1` stays rejected, covered above.)
+    doc = parse("[a.b.c]\n[a]\nb.z = 1\n")
+    assert doc.unwrap() == {"a": {"b": {"c": {}, "z": 1}}}
+    assert doc.as_string() == "[a.b.c]\n[a]\nb.z = 1\n"
+
+
 def test_set_value_on_out_of_order_table_with_empty_concrete_part() -> None:
     # A super table defined after its sub-table (the "defining a super-table
     # afterward is ok" spec example) leaves an empty concrete `[x]` part.

tomlkit/container.py

@@ -426,7 +426,9 @@ def _validate_table_candidate(self, current: Table, candidate: Table) -> None:
                 existing = current.value.item(k)
                 if isinstance(existing, (Table, AoT)) != isinstance(v, (Table, AoT)):
                     raise KeyAlreadyPresent(k)
-                if k.is_dotted():
+                if k.is_dotted() and not (
+                    isinstance(existing, Table) and existing.is_super_table()
+                ):
                     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():