Unaceptable and unprofessional towards clients

They have taken double payments more than once Its been on going for more tha 6 months that i am strugling to get my money that they owe me. I provide them everytime what they ask then then all of a sudden need completely other documents This is un acceptable as i was a loyal client that was always up to date and due to their error taking a big amount extra now they keep on having exuses to pay my money back As per their cover they are not bad but their after sales is unaceptable!!!!!!!