uniquify cols for FK table object

uniquify cols for FK table object

Fixed issue where using a directive such as ``op.create_foreign_key()`` to
create a self-referential constraint on a single table where the same
column were present on both sides (e.g. within a composite foreign key)
would produce an error under SQLAlchemy 2.0 and a warning under SQLAlchemy
1.4 indicating that a duplicate column were being added to a table.

Change-Id: I2a8f5d8def2714792bffcdfb8bf88a5080ec8ce7
Fixes: #1215