The back of the freezer has to be below freezing to work properly. If there is Ice/frost build up on the back, it is caused by moisture in the freezer freezing on the back. Check the door seal to insure that it is not damaged. If the door is working properly, than it is most likely just caused by opening the freezer door in very humid environment. When it is humid, you need to minimize the time the freezer door is open.
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