Wrong number of items in pagination if not unique (and should unique be enforced?)

## Description By default, the pagination query methods enforce `.unique()` on the session/execute level. Since limit/offset is done beforehand at the query level, if the retrieved elements are not unique, the selected objects will not reflect the uniqueness/distinct filter and the query will return less elements than defined by `per_page`. See the code below. ``` class SelectPagination(Pagination): """Returned by :meth:`.SQLAlchemy.paginate`. Takes ``select`` and ``session`` arguments in addition to the :class:`Pagination` arguments. .. versionadded:: 3.0 """ def _query_items(self) -> list[t.Any]: select = self._query_args["select"] select = select.limit(self.per_page).offset(self._query_offset) # **The pagination limits are applied to the select statement** session = self._query_args["session"] return list(session.execute(select).unique().scalars()) # **But only here the `.unique()` is applied** (...) ``` On a side-note, I don't think `.unique()` should be enforced at this level; I believe this should be achieved manually by the user at the select statement level or, at least, be optional based on an attribute in the `Pagination` class. For me, at least, this is highly undesireable because 1) I might not want this filter to be included in my query, and 2) it might obfuscate problems I might have in my query. Please consider adding this flag for now for users to opt-out of this (to avoid breaking deployments) and possibly remove this filter altogether in a future major (breaking) change. ## Fix - Apply `.distinct()` in the `select` statement and remove `. unique()` from the `session.execute` - (Optional) Make this unique filter optional ## Environment - Python version: 3.11 - Flask-SQLAlchemy version: main branch ([3e3e92b](https://github.com/pallets-eco/flask-sqlalchemy/commit/3e3e92ba557649ab5251eda860a67656cc8c10af)), latest release 3.1.1 - SQLAlchemy version: 2.0.36