If you can't work it out with the GC, your only real option is to refuse to pay. If they are not willing to let it go, they could sue you for that invoice; in the lawsuit, they will have to prove that it is a new charge and you can in turn provide evidence that it is duplicative; a court will decide who is right, and order whether or not you have to pay. If it's not worth the time, stress, etc. of having to potentially defend a lawsuit (i.e. if it's not that large a bill), you may be best off paying it even if you feel it duplicative.