check that two CTEs aren't just annotated forms of the same thing

check that two CTEs aren't just annotated forms of the same thing

Fixed issue where using :func:`_orm.aliased` around a :class:`.CTE`
construct could cause inappropriate "duplicate CTE" errors in cases where
that aliased construct appeared multiple times in a single statement.

Fixes: #12364
Change-Id: I9625cd83e9baf5312cdc644b38951353708d3b86
(cherry picked from commit 42ddb1fd5f1e29682bcd6ccc7b835999aafec12e)