ExecuteTruncate quirk: expects a unique list of relations

Hi,

Consider this simple case:

postgres=# TRUNCATE foo, foo;
ERROR: cannot TRUNCATE "foo" because it is being used by active queries in
this session

The above occurs because the ExecuteTruncate() function invokes
truncate_check_rel() in a loop. Since the same table name appears twice, the
rd_refcnt for table "foo" is bumped up to 2, causing the above failure.

We might want to add a step to ExecuteTruncate(), or whatever calls it, to
make the list unique.

Regards,
Nikhils
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