Allow same-key updates with duplicate conversion checks

[muneikh]

With box_duplicates="error", reassigning an existing converted key incorrectly raises a duplicate-attribute error. With "warn", the same update produces a warning. For example, b = Box({"a b": 1}, box_duplicates="error"); b["a b"] = 2 fails because the existing safe alias is treated as a different conflicting key.

Only flag a conversion duplicate when the alias points to a different original key. Tests cover dictionary and attribute updates under both policies, then verify that a genuinely different colliding key still raises or warns. The changelog and author list are updated per the contribution guide.

Validation: all four new cases fail before the fix; the complete suite passes after it on Python 3.13 (163 tests) with the Cython build and required format backends. The configured Black 24.10 whole-tree check and Mypy (8 source files) pass; git diff --check passes. The PR targets the required develop branch.