How do I Take Care of This

I recently made a settlement with a CA for an old credit card account. I paid about $3000 less than was owed. I have in writing from them that they would report this account as settled in full. I paid them as agreed. This was about 3 months ago. The other day I got a letter from a different CA and it looks like they are trying to collect the remaining balance from the same account. I was wondering what my next course of action should be.