### What changes were proposed in this pull request?
Currently in Spark one could redefine a window. For instance:
`select count(*) OVER w FROM tenk1 WINDOW w AS (ORDER BY unique1), w AS (ORDER BY unique1);`
The window `w` is defined two times. In PgSQL, on the other hand, a thrown will happen:
`ERROR: window "w" is already defined`
### Why are the changes needed?
The current implement gives the following window definitions a higher priority. But it wasn't Spark's intention and users can't know from any document of Spark.
This PR fixes the bug.
### Does this PR introduce _any_ user-facing change?
Yes.
There is an example query output with/without this fix.
```
SELECT
employee_name,
salary,
first_value(employee_name) OVER w highest_salary,
nth_value(employee_name, 2) OVER w second_highest_salary
FROM
basic_pays
WINDOW
w AS (ORDER BY salary DESC ROWS BETWEEN UNBOUNDED PRECEDING AND 1 FOLLOWING),
w AS (ORDER BY salary DESC ROWS BETWEEN UNBOUNDED PRECEDING AND 2 FOLLOWING)
ORDER BY salary DESC
```
The output before this fix:
```
Larry Bott 11798 Larry Bott Gerard Bondur
Gerard Bondur 11472 Larry Bott Gerard Bondur
Pamela Castillo 11303 Larry Bott Gerard Bondur
Barry Jones 10586 Larry Bott Gerard Bondur
George Vanauf 10563 Larry Bott Gerard Bondur
Loui Bondur 10449 Larry Bott Gerard Bondur
Mary Patterson 9998 Larry Bott Gerard Bondur
Steve Patterson 9441 Larry Bott Gerard Bondur
Julie Firrelli 9181 Larry Bott Gerard Bondur
Jeff Firrelli 8992 Larry Bott Gerard Bondur
William Patterson 8870 Larry Bott Gerard Bondur
Diane Murphy 8435 Larry Bott Gerard Bondur
Leslie Jennings 8113 Larry Bott Gerard Bondur
Gerard Hernandez 6949 Larry Bott Gerard Bondur
Foon Yue Tseng 6660 Larry Bott Gerard Bondur
Anthony Bow 6627 Larry Bott Gerard Bondur
Leslie Thompson 5186 Larry Bott Gerard Bondur
```
The output after this fix:
```
struct<>
-- !query output
org.apache.spark.sql.catalyst.parser.ParseException
The definition of window 'w' is repetitive(line 8, pos 0)
```
### How was this patch tested?
Jenkins test.
Closes#30512 from beliefer/SPARK-28645.
Lead-authored-by: gengjiaan <gengjiaan@360.cn>
Co-authored-by: beliefer <beliefer@163.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
e43255051c