Crash when partition column specified twice

Noticed that a crash occurs if a column is specified twice when creating a
partition:

create table p (a int) partition by list (a);

-- crashes
create table p1 partition of parent (
a not null,
a default 1
) for values in (1);

The logic in MergeAttributes() that merged partition column options with
those of the parent didn't properly check for column being specified twice
and instead tried to delete the same ColumnDef from a list twice, causing
the crash.

Attached fixes that.

Added to the open items list.

Thanks,
Amit