I paid a bill to collections, now another company is sending me bills like I never paid!?

Here's the scoop:

I had a credit card that I let lapse due to poorness. After awhile, it went to collections, of course. I got one of those "pay in full yada yada" letters and by chance had gotten my tax return and used it to pay the stupid thing. This was to a company, I'm pretty sure, called Comenity Bank. I sent them a check for the whole bill with a note saying I expected a reciept saying I paid in full. I never got this reciept which did not make me happy but what was I gonna do?

Now I'm getting letters from ANOTHER company saying "you can pay half on this and we'll be square" from some place called Hudson.

I managed to search my old e-statements for my bank and sure enough there's my check #1104 for the full amount but, of course, it doesn't show who I wrote it to.

Do I have any recourse? I don't even know who to call. Will my bank be of any help since this was back in February and it's now September? Can Comenity help? I can't find anything on their website.

Should I call Hudson first?