I want to prevent the insert duplicates values in table to solve this issue i use the syntax "WHERE NOT EXISTS" but not working so please what is the correct syntax to solve this problem.
INSERT INTO [JPCustomer] ([CustomerID ],
                           [JPID],
                           [Frequency],
                           [StartWeek],
                           [sat],
                           [sun],
                           [mon],
                           [tue],
                           [wed],
                           [thu],
                           [fri],
                           [VisitOrder],
                           [ModifiedOn],
                           [ModifiedBy],
                           [CreatedOn],
                           [Createdby],
                           [RecordSource],
                           [IsPotential])
select cu.CustomerNo,
       jp.ID,
        4,
       1,
        1,
        1,
        1,
        1,
        1,
        1,
        1,
        NULL,
        NULL,
        NULL,
        NULL,
        NULL,
        0,
        0 
from CUSTOMERNO# cu
join SalesmanNo# sa on cu.OCCURRENCE = sa.OCCURRENCE
join JourneyPlan JP on jp.AssignedTO = sa.SalesmanNo
WHERE NOT EXISTS (select j.CustomerID,j.JPID from JPCustomer j)