It seems the duplicates are caused by the task_change_time from the job_tasks table. Even though the table definition shows task_change_time is the time last time the task was modifed.. But it is capturing different times and it is SCD type 2 table. I updated the query. Can you please use this query. probably you can raise a Databricks ticket to see the reason to different  task_change_time eventhough the task is not updated.

WITH workspace AS (
  SELECT
    account_id,
    workspace_id,
    workspace_name,
    workspace_url,
    status,
    workspace_name
  FROM system.access.workspaces_latest
),
usage_with_ws_filtered_by_date AS (
  SELECT
    u.*,
    w.workspace_name,
    w.workspace_url
  FROM system.billing.usage u
  INNER JOIN workspace w ON u.workspace_id = w.workspace_id
  WHERE u.billing_origin_product = 'JOBS'
    AND u.usage_date BETWEEN DATE_ADD(CURRENT_DATE(), -30) AND CURRENT_DATE()
),
task_usage AS (
  SELECT
    u.workspace_id,
    u.workspace_name,
    u.workspace_url,
    u.usage_metadata.job_id,
    u.usage_metadata.job_run_id AS run_id,
    t.task_key,
    t.change_time AS task_change_time,
    u.usage_start_time,
    u.usage_end_time,
    u.usage_quantity,
    u.sku_name,
    u.identity_metadata.run_as AS user,
    u.usage_date
  FROM usage_with_ws_filtered_by_date u
  INNER JOIN system.lakeflow.job_tasks t
    ON u.workspace_id = t.workspace_id
    AND u.usage_metadata.job_id = t.job_id
),
task_costs AS (
  SELECT
    tu.*,
    lp.pricing.default AS unit_price,
    tu.usage_quantity * lp.pricing.default AS cost
  FROM task_usage tu
  LEFT JOIN system.billing.list_prices lp
    ON tu.sku_name = lp.sku_name
    AND tu.usage_start_time >= lp.price_start_time
    AND (tu.usage_end_time <= lp.price_end_time OR lp.price_end_time IS NULL)
    AND lp.currency_code = 'USD'
)
SELECT DISTINCT
  tc.workspace_id,
  tc.workspace_name,
  tc.workspace_url,
  tc.user,
  tc.job_id,
  tc.run_id,
  tc.task_key,
  tc.usage_start_time,
  tc.usage_end_time,
  tc.usage_date,
  SUM(tc.cost) AS total_cost
FROM task_costs tc
GROUP BY
  tc.workspace_id, tc.workspace_name, tc.workspace_url, tc.user, tc.job_id, tc.run_id,
  tc.task_key, tc.usage_start_time, tc.usage_end_time, tc.usage_date
ORDER BY
  tc.usage_date DESC, tc.workspace_id, tc.job_id, tc.run_id, tc.task_key