Subject: Fix a reference error for window functions: In the function 'find_window_functions', the deduplication logic should be removed

From: Meng Zhang <[email-masked]>
Date: Sun, 25 Jan 2026 12:09:27 +0800

The deduplication logic won't cause an error when the result of this
function is only used in `select_active_windows`.
But when the result is used in `optimize_window_clauses`, it will cause the
`winref` field of a certain window function to not be modified in the new
window.