And another with the same technique:
CREATE TABLE #Template (
    TaskName VARCHAR(20),
    TemplateName VARCHAR(20)
);
INSERT INTO #Template (TaskName, TemplateName)
SELECT 'Task1', 'Template 1' UNION ALL
SELECT 'Task2', 'Template 1' UNION ALL
SELECT 'Task3', 'Template 1' UNION ALL
SELECT 'Task4', 'Template 1'

SELECT *
FROM #Template;

CREATE TABLE #TasksByPlan (
    TaskName VARCHAR(20),
    TemplateName VARCHAR(20),
    PlanName VARCHAR(20)
);
INSERT INTO #TasksByPlan (TaskName, TemplateName, PlanName)
SELECT 'Task1', 'Template 1', 'Plan1' UNION ALL
SELECT 'Task2', 'Template 1', 'Plan1' UNION ALL
SELECT 'Task3', 'Template 1', 'Plan1' UNION ALL
SELECT 'Task1', 'Template 1', 'Plan2' UNION ALL
SELECT 'Task2', 'Template 1', 'Plan2' UNION ALL
SELECT 'Task3', 'Template 1', 'Plan2' UNION ALL
SELECT 'Task4', 'Template 1', 'Plan2' UNION ALL
SELECT 'Task1', 'Template 1', 'Plan3' UNION ALL
SELECT 'Task2', 'Template 1', 'Plan3';

SELECT TBP.TemplateName, TBP.PlanName, TBP.TaskName
FROM #TasksByPlan AS TBP
ORDER BY TBP.TemplateName, TBP.PlanName, TBP.TaskName;

WITH TasksByPlan AS (

    SELECT DISTINCT TBP.TemplateName, TBP.PlanName
    FROM #TasksByPlan AS TBP
),
    TaskTemplate AS (

        SELECT DISTINCT T.TemplateName, T.TaskName
        FROM #Template AS T
)
INSERT INTO #TasksByPlan (TaskName, TemplateName, PlanName)
SELECT TT.TaskName, TT.TemplateName, TBP.PlanName
FROM TaskTemplate AS TT
    INNER JOIN TasksByPlan AS TBP
        ON TT.TemplateName = TBP.TemplateName
        AND TBP.PlanName + TT.TaskName NOT IN (
            SELECT TBP2.PlanName + TBP2.TaskName
            FROM #TasksByPlan AS TBP2
            WHERE TBP2.TemplateName = TBP.TemplateName
            );

SELECT TBP.TemplateName, TBP.PlanName, TBP.TaskName
FROM #TasksByPlan AS TBP
ORDER BY TBP.TemplateName, TBP.PlanName, TBP.TaskName;

DROP TABLE #TasksByPlan;
DROP TABLE #Template;