collection agency

i have two different collection agencies trying to collect for the same debt they both have sent me a validation letters. the weird thing is the second collection agency has the first collection on their letter and as previous creditor. i called the 1st collection agency and the rep stated they haven't sold my account to anyone, so i then told her about the second collection agency and she stated they don't know anything about the second agency. The second problem is a had set up a payment arrangement with the second agency online before i called the first agency to verify if they had sold the account. I'm really not sure what to do now, the second agency cost is higher by $400.00 and i would rather pay the first agency. please help