Most people believe that when they've paid their portion of a medical bill it's over and done with. Nothing could be further from the truth!
Insurance companies do internal audits all the time to check for claims that they shouldn't have paid, or to check for claims they over paid. In some cases such as eligibility, there's no time limit as to how far back they will check claims. In other cases the time limit will be dictated by state laws.
Providers may also have internal audits in place to check for claims that were underpaid or denied line items. Speaking from personal experience, it's not at all uncommon for providers to notify the insurance company of such underpayments and then have to haggle their case for two or three years. Providers will also audit for posting mistakes as well which can cause you to be re-billed for the correct amount when the mistakes are corrected in their system.
What happens in these cases is that when the insurance company re-reviews a claim, they also review your co-insurance and deductibles for the CURRENT time period. So while you may have met your deductible in 2010 it's now 2012 and you have not and you could owe additional money. In this scenario you could be getting a bill for something you thought you paid two years ago!
So what does a patient do in these scenarios to avoid being paying for something that they shouldn't?
First, keep a copy of your EOB's for at least two years and longer if possible so that you have record of the original record of how they processed the claim. Most insurance companies will purge their records online after 18 months. In these cases it's impossible to get a duplicate copy of an EOB unless you jump through a lot of hoops. If you do need an original copy of an EOB that they say they've purged, ask them if they would have it if you were an IRS agent doing an audit -- in that case they would have it!
Second, it's a good idea to keep proof of your payments on your medical bills AND your insurance premiums for at least five years. You need to have a way to prove that you paid your bill or that you had coverage should any errors occur -- and they often do!
Third, get familiar with your state's laws regarding time limits to file claims and bill patients. Most states have laws in place because of scenarios such as this. If you know the laws you'll know whether or not you're being billed for something you shouldn't be.
Fourth, verify with your provider why you're being billed at such a late date for something you've already paid for. Are they billing you because they billed you incorrectly the first time or are they re-billing you because the insurance company re-processed your claim?
Last, verify with your insurance company if they reprocessed the claim and if so why? If they're processing an old claim against current benefit levels, ask them why you're being penalized for THEIR mistake? Don't be afraid to argue with them over this point.