paying two different companies

i bought a laptop at Nashua mobile in 2013, i have been paying since then, torwards the end of 2014 i skipped few months of payments, the on then 20th Dec 2014 i paid R1000 to Nashua mobile Nedbank account, two days later i recieved an sms from Saya recoveries telling me that they took over Nashua Mobile account and the told mei owe R2000, which i already paid half of it at Nashua Mobile account, then i paid the balance at Saya recoveries. few days later i recieved a call from MTN telling me they have taken over Nashua mobile account. i told them i already paid and i forwarded them the proof of payment and the letter that states that i paid Nashua mobile in full. now MTN is harrasing me asking me to pay