Generally, you should not. When mating two CF flanges, the flanges should be tightened until the flange faces are touching (known as "metal to metal"). If this is done, trying to reuse that gasket has a zero percent chance of being leak tight. Some users tighten CF flanges partially, hope they are leak tight, and will attempt to reuse the copper gasket after dismantling and retightening the flanges. I can work, but it is not guaranteed or recommended.