insert ignore on duplicate key update returns mistake dup rows

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
CREATE TABLE `t` (
`id` int NOT NULL,
`v` int DEFAULT NULL,
PRIMARY KEY (`id`) /*T![clustered_index] CLUSTERED */
)

insert ignore into t values(1, 10), (2, 20) on duplicate key update v=v+1;
```

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

The message should return 0 duplicates because the table is empty

```
> insert ignore into t values(1, 10), (2, 20) on duplicate key update v=v+1;
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
```

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

The message returns `2` duplicates.

```
> insert ignore into t values(1, 10), (2, 20) on duplicate key update v=v+1;
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 2 Warnings: 0
```

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

master