Regression in 0.15.1: valid out-of-order child table raises KeyAlreadyPresent #571

Summary

Tomlkit 0.15.1 rejects a valid document when a table contains an out-of-order
child, its concrete parent is declared afterward, and another table separates a
later sibling child.

This is a regression: tomllib and Tomlkit 0.13.3, 0.14.0, and 0.15.0 all
accept the document.

Minimal reproduction

import tomlkit

source = """\
[tool.ruff]
[tool.ruff.lint.a]
[tool.ruff.lint]
[[tool.poetry.source]]
[tool.ruff.lint.b]
"""

tomlkit.loads(source)

Actual result

tomlkit.exceptions.ParseError: Key "lint" already exists. at line 5 col 0

Expected result

The document parses successfully, matching tomllib:

{
    "tool": {
        "ruff": {"lint": {"a": {}, "b": {}}},
        "poetry": {"source": [{}]},
    }
}

Each header in the minimized example is necessary to trigger the failure. The
array-of-tables is not special: replacing [[tool.poetry.source]] with any
intervening table header also reproduces it, while removing the intervening
header makes it pass.

Regression boundary

Reproduced on Python 3.13.8 and 3.14.2:

• stdlib tomllib: passes
• Tomlkit 0.13.3: passes
• Tomlkit 0.14.0: passes
• Tomlkit 0.15.0: passes
• Tomlkit 0.15.1 and current master: fail

Suspected root cause

The regression appears to come from the concrete+super validation added in
#530 / commit d3c76f0.

When [tool.ruff.lint.b] is appended, the existing lint entry is represented
by OutOfOrderTableProxy, while the candidate is a Table.
_validate_table_candidate() checks:

isinstance(existing, (Table, AoT)) != isinstance(v, (Table, AoT))

The proxy is table-like but fails the left-hand isinstance check, so valid
input is reported as a type conflict and raises KeyAlreadyPresent("lint").