turn off deduping for col expressions

turn off deduping for col expressions

Fixed ORM issue where column expressions passed to ``query()`` or
ORM-enabled ``select()`` would be deduplicated on the identity of the
object, such as a phrase like ``select(A.id, null(), null())`` would
produce only one "NULL" expression, which previously was not the case in
1.3. However, the change also allows for ORM expressions to render as given
as well, such as ``select(A.data, A.data)`` will produce a result row with
two columns.

Fixes: #6979
Change-Id: I4dd59d4c7b1baa711b686379eb959f87c44841c4