dbt Snapshot Failing (ERROR: 100090 (42P18): Duplicate row detected during DML action)

So we have a table called dim_merchant.sql and a snapshot of this table called dim_merchant_snapshot.

{% snapshot dim_merchant_snapshot %}

    {{
        config
        (
          target_schema='snapshots',
          unique_key='id',
          strategy='check',
          check_cols='all'
        )
    }}

select * from {{ ref('dim_merchant') }}

{% endsnapshot %}

We never had a trouble with it but since yesterday, we ran into failure in running the snapshot with the following error message:

Database Error in snapshot dim_merchant_snapshot (snapshots/dim_merchant_snapshot.sql)
100090 (42P18): Duplicate row detected during DML action

The error is happening during this step of the snapshot:

On snapshot.analytics.dim_merchant_snapshot: merge into "X"."SNAPSHOTS"."DIM_MERCHANT_SNAPSHOT" as DBT_INTERNAL_DEST
    using "X"."SNAPSHOTS"."DIM_MERCHANT_SNAPSHOT__dbt_tmp" as DBT_INTERNAL_SOURCE
    on DBT_INTERNAL_SOURCE.dbt_scd_id = DBT_INTERNAL_DEST.dbt_scd_id

    when matched
     and DBT_INTERNAL_DEST.dbt_valid_to is null
     and DBT_INTERNAL_SOURCE.dbt_change_type in ('update', 'delete')
        then update
        set dbt_valid_to = DBT_INTERNAL_SOURCE.dbt_valid_to

    when not matched
     and DBT_INTERNAL_SOURCE.dbt_change_type = 'insert'

We realized that some values were being inserted and updated twice in the snapshot (since yesterday) and that caused the failure of our snapshot but we are not sure as to why.

Note that the id key on dim_merchant is tested for its uniqueness and there are no duplicated of it. Meanwhile, the snapshot table contains duplicate after our first snapshot run (that doesn't cause any failure), but the subsequent runs on the snapshot table infected with duplicates are failing.

We just recently updated dbt from 0.20.0 to 1.0.3, but we didnt find any change in the snapshot definition between these versions.

SETUP:

dbt-core==1.0.3,
dbt-snowflake==1.0.0,
dbt-extractor==0.4.0,
Snowflake version: 6.7.1 

Thanks !