Not closing a ********** account

In April last year, I got a call from a lady from MTN saying that someone had taken up a contract with them, and i owed them money. I followed their instructions and sent them the affidavits and ID copies as requested. To my surprise, a few months later, I got a call from MBD, who informed me that MTN had handed the account over to them, again I followed their instructions, and redid the affidavits, and submited them as requested, the account was subsequently removed from my credit record last October. Unbelievably though, this morning I got a call from SSDA (Shapiro Shaik Defries & Associates) saying that MTN has now handed over the same account to them, and now the also want me to go through the same process a third time for an account that I never owned to begin with. Even if i were to do this again, how long till another debt collector calls me about the same account?