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

[Meng Zhang]

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.

0001-Fix-a-reference-error-for-window-functions.patch
Description: Binary data