Duplicate column names exist in `show grants` if current user set roles

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql

-- root
use test;
CREATE USER u;
CREATE USER r1;
CREATE USER r2;
CREATE TABLE t (c1 int, c2 int);

GRANT select(c1, c2) ON test.t TO u;
GRANT select(c1, c2) ON test.t TO r1;
GRANT select(c1, c2) ON test.t TO r2;
GRANT r1 TO u;
GRANT r2 TO u;
SET DEFAULT ROLE ALL to u;

-- login as u
show grants;
```

### 2. What did you expect to see? (Required)

```sql
GRANT USAGE ON *.* TO 'u'@'%';
GRANT SELECT(c1, c2) ON `test`.`t` TO 'u'@'%';
GRANT 'r1'@'%', 'r2'@'%' TO 'u'@'%';
```

### 3. What did you see instead (Required)

```sql
GRANT USAGE ON *.* TO 'u'@'%';
GRANT SELECT(c1, c2, c1, c2, c1, c2) ON `test`.`t` TO 'u'@'%';
GRANT 'r1'@'%', 'r2'@'%' TO 'u'@'%';
```

### 4. What is your TiDB version? (Required)

master