Handed over to debt collectors for a paid up loan

I took out a loan with capfin last year and I lost my job, couldn't pay then I was handed over to debt collectors (black fox credit management). I made arrangements to pay when I got a new job of which I settled my loan immediately. However my name has not been cleared yet and I got another phone call from a different debt collected by the name of Sadler saying I must settle a loan that I have with capfin. I have contacted your useless legal department numerous times and even forwarded proof of settlement yet the had been no resolve to the matter 7 months after I have settles the loan. This is destroying my credit profile massively.