For what I know about RingCT it use fake outputs to mask the amount that is transaction between address's, how do we know that the fakeoutputs are correct and that is noone messing with them?
1 Answer
RingCT does not create fake outputs. Here is a good dumbed-down explanation of its principle: ELI5: How does RingCT work?
Therefore there is no difference between "normal" transactions and RingCT ones in terms of double-spend prevention: it is handled in the exact same way using key images.