I received a phone call and a letter from a debt collection agency for an old debt that I already paid to the debt collection agency that previously owned it, 7 years ago. The only proof I still have are my bank statement and a tax form 1099-C that I received because the collection agency settled the debt with me. I don't want to send copies of these documents to this new collection agency because they contain sensitive information and I'm unfamiliar with this company. I also don't want to just ignore it because I've worked hard to rebuild my credit and I'm afraid the debt will end up back on my credit report as new. How should I proceed?