Here's some better example of where this is really a problem (the initial queries just illustrated what we are talking about).

select a from A a order by a

is valid.

We want (its a long standing feature request) to also allow:

select a+1 as b from A a order by b

The first query uses the from-clause declared alias as an expression in the order by. The second query uses the select-clause declared alias as an expression in the order by.

So now take a query that allows the duplicate alias:

select a.something as a from A a

You said you want to allow this. But now consider:

select a.something as a from A a order by a

What does a in order by a mean?