duplicate column names in a table alias should be prevented.

[nicktobey]

Repro steps:

drop table if exists ab;
create table ab (a int, b int);
insert into ab values (1, 1), (2, 2);

drop table if exists bc;
create table bc (b int, c int);
insert into bc values (1, 1), (2, 2);

SELECT * FROM (SELECT * FROM ab join bc on ab.b != bc.b) as abc;

SELECT b FROM (SELECT * FROM ab join bc on ab.b != bc.b) as abc;

MySQL results:

ERROR 1060 (42S21) at line 10: Duplicate column name 'b'

Dolt results:

+---+---+---+---+
| a | b | b | c |
+---+---+---+---+
| 2 | 2 | 1 | 1 |
| 1 | 1 | 2 | 2 |
+---+---+---+---+

+---+
| b |
+---+
| 2 |
| 1 |
+---+

In this example, the table alias has two columns named b, and one of them appears to be selected arbitrarily. We should be consistent with MySQL and reject this.

(Mirror of dolthub/go-mysql-server#1881)