- Duplicate items in a list-based collection will

- Duplicate items in a list-based collection will
be maintained when issuing INSERTs to
a "secondary" table in a many-to-many relation.
Assuming the m2m table has a unique or primary key
constraint on it, this will raise the expected
constraint violation instead of silently
dropping the duplicate entries. Note that the
old behavior remains for a one-to-many relation
since collection entries in that case
don't result in INSERT statements and SQLA doesn't
manually police collections. [ticket:1232]