If you’ve ever stayed at a hospital, chances are you’ve been the victim of double billing. A common practice among hospitals, double billing is the act of billing you twice for the same procedure or supply. Sometimes, you are double billed as a result of a clerical error, a decimal in the wrong place, an extra zero on your final bill. But, more often than not you are double billed for one reason: hospitals know they can get away with it.

Insurance companies generally don’t catch duplicate charges, they reject claims or pay portions of them often without searching for their validity. This means one thing, fighting double billing falls on your shoulders.

Before paying a hospital or doctor bill, it’s important to look for duplicate charges. Ask for an itemized list and go through it with a fine tooth comb. In particular, pay attention to the following:

Mistakes that Cost You: Sometimes you may have the same test or procedure twice. An x-ray wasn’t developed clearly enough, a blood test was lost in the lab. If you have the same procedure twice, you will often be charged twice. Instead of paying these, contest them. It’s not your fault if a procedure was poorly performed or if a test result was lost.

Bundles that are Unwrapped: “Unbundling” is a common practice in hospitals. It is the act of taking a collection of tests or procedures and charging for each one separately. For instance, if you go in to have your appendix removed, everything involved in the surgery should be bundled together. You shouldn’t have to pay separately for stitches or IV fluids as they should be included in the cost of the procedure. If you are charged for these individual items, it is the act of un-bundling. Unbundling increases the cost of your hospital bill. This is why hospitals often do it.

Packages that are Unpackaged: When viewing your hospital bill, it’s important to look for anything that involves a tray, a package, or a pack. Hospitals have a knack for charging you for both the entire package, and then the package’s individual items, such as gauze. This is a form of double billing. You shouldn’t have to pay for both the pack and its contents.

Doctors You Don’t Actually Know: If you receive a bill from a doctor you’ve never heard of, you need to take action. Start by finding out who the doctor is and what services he provided to you. Give him a call and request documentation. If he doesn’t have documentation of his service to you, then he doesn’t have a right to ask for payment.

Remember the 20 Percent Rule: When you go under the knife, there is often an assistant surgeon in the operating room. You will probably not realize this until you receive a bill from him. If you do receive a bill, make sure that it is not more than 20 percent of what the main surgeon was charging. Most insurance companies stipulate that assistants aren’t allowed to charge more than 20 percent of the primary surgeon’s fee.

Be Aggressive, but Polite: A hospital probably won’t be thrilled that you’ve uncovered their duplicate charges. For this reason, it’s important to be aggressive and persistent. But, also be polite. You don’t want to come across as rash or unreasonable. Be calm and cool and state your case in a clear manner.

It may seem as if hospital bills leave you with little leverage - if your appendix erupts you can’t exactly just say “no thanks” to a surgery. But, remember that you always pay the bill after the fact. You can have the procedure, and then deal with the bill at a later date. When this later date comes, look very carefully for duplicate charges and contest any that you see. The hospital may be trying to pull the wool over your eyes, but don’t let them. Being informed and being prepared will save you from paying twice for the exact same thing.