Receiving two letters for the same debt is not uncommon. Here's why. . .
The original creditor determined that the account was no longer collectable and decided to "sell" the account to an account receivable recovery specialist [ARRS] (aka debt collector). The account went into a pool of accounts and the process of collecting was in play. The first ARRS also used a third party (forth party to you) to collect as well. This creates the two letter conundrum. As well, this also gives rise to multiple entries of collection on your credit report - it is simply the incomplete reliquishing of accounts between collectors. The more collectors involved, the more letters and the more entries on your credit report.
Once you are financially able, talk to the original creditor about reinstating the account to settle it. If this is not possible, then reach the first "third-party" which contacted you as they have the rights of the original creditor. Make payment arrangements that do not involve the use of wiring your money from your personal account or sending a check (i.e., use a money order) to satisfy the payment.
Once you've received the "paid in full" letter, wait 30 days to request your credit report. You should see the account written off from the original creditor, the balance bought to zero on the second creditor, and also the other party's balance brought to zero (sometimes the entry goes away altogether). If the entries are not correct, notify the credit reporitng agencies to make the correction.
Keep copies of everything including the money order. Good luck and I hope things get better for you.