[Bug] Using snapshots with `check` strategy sometimes creates duplicate rows

### Is this a new bug in dbt-core?

- [x] I believe this is a new bug in dbt-core
- [x] I have searched the existing issues, and I could not find an existing issue for this bug

### Current Behavior

We use dbt's snapshot functionality with the [`check` strategy](https://docs.getdbt.com/docs/build/snapshots?version=1.12#check-strategy) to create snapshots from some source tables to track changes. The reason for the check strategy is that we only need to track one column out of many, and won't want a new row each time other non-tracked columns are updated.

When an existing row with the check column is updated from `null` to any non-null value, two rows identical rows are inserted instead of just one. These two rows have the same values in snapshot metadata fields as well (`dbt_scd_id`, `dbt_updated_at`, etc.)

### Expected Behavior

Snapshot should only insert one row on source change.

### Steps To Reproduce

with a snapshot model config like:

```sql
{% snapshot cards %}

{{
config(
unique_key='id',
strategy='check',
check_cols=['maximum_spend_amount']
)
}}

select
id
, maximum_spend_amount
, created_at
, updated_at
from {{ source('source_db', 'cards') }}

{% endsnapshot %}
```

run `dbt snapshot` when with row that has `null` for `maximum_spend_amount` column.
update same row's `maximum_spend_amount` with a non-null value, run `dbt snapshot`
the same row is created twice in the snapshot.

### Relevant log output

```shell

```

### Environment

```markdown
- OS: Debian GNU/Linux 12 (bookworm)
- Python: 3.12
- dbt: 1.9.1
```

### Which database adapter are you using with dbt?

snowflake

### Additional Context

_No response_