I received a letter from a collection agency back in June, which I disputed and requested validation for. I did not receive an reply back from them. Today I received a letter from a different agency for the same debt. I understand that sometimes if an agency cannot validate a debt, they will sell the debt to another agency. My question is, how long can this go on for? If the agencies keep selling it to other agencies, I have to keep disputing it - which is a huge waste of time!