Creating an incremental model using the MERGE strategy with a unique key. The unique key to merge on is a surrogate key of columns from the table.

When I run this incremental model I get the error: Duplicate row detected during DML action.

I have confirmed this key is unique for incrementation so I am very confused by this error.

-- Model Configuration
{{  
    config(
        materialized = 'incremental',
        unique_key = ['loan_history_key']
    )
}}

-- Import dependencies
-- depends_on: {{ ref('dim_loans') }}
-- depends_on: {{ ref('dim_cifs') }}
-- depends_on: {{ ref('fact_loan_details') }}
-- depends_on: {{ ref('fact_loan_payments') }}
-- depends_on: {{ ref('dim_dates') }}

-- Select data and generate surrogate key
WITH loan_histories AS (
    SELECT
        dim_loans.loan_key AS loan_key,
        dim_dates.date_key AS as_of_date_key,
        -- Add other necessary fields
    FROM
        {{ ref('dim_loans') }} AS dim_loans
    JOIN
        {{ ref('dim_dates') }} AS dim_dates
        ON dim_loans.some_date = dim_dates.date
    -- Include other necessary joins
)

SELECT
    {{ dbt_utils.generate_surrogate_key(['loan_key', 'as_of_date_key']) }} AS loan_history_key,
    *
FROM
    loan_histories

-- Handle incremental updates
{% if is_incremental() %}
WHERE  
    TO_DATE(TO_CHAR(as_of_date_key), 'YYYYMMDD') >= current_date - interval '3 day'
{% endif %}