This is my area. I work for an insurance company resolving just this sort of problem (not Aetna). First, how much was the total bill? It looks like $6500? Or was it $3500?
If I am understanding your claim, the hosp bill was $6500 of which you paid $3000 up front. Aetna has nothing to do with that, and has no way of knowing that you paid, unless (as very rarely happens) the hosp bothered to note on the claim form that you had paid. I see that once in a thousand claims, maybe. Usually what happens is that the hosp sends the claim to us, we process and send out an EOB, and a check for whatever is left over after the deductible.
So what I imagine happened is that Aetna got a claim, processed it and sent an EOB to your hosp, showing that you owed $3000, and they paid the remainder. Then, your hospital's billing dept got ahold of the eob and fired off a bill to you, without bothering to check their own accounting department's records to see if you had paid anything already. Your problem probably isn't with Aetna, but with hospital billing. Call them, and start faxing them copies of your receipt. Call them again and again until you get someone to admit that you already paid. Once you have that admission on file, get ahold of the bill collector and tell them the bill is paid. If they continue to harass you, then it is time to take a complaint to the state. You will also need to get your credit records updated.