Let's say I have three outputs that I own: 1st output with 1 Monero, 2nd output with 1 Monero, 3rd output with 100 Monero.
The logic of range proofs supposes that the amount of input Monero equals to the amount of output Monero.
What prevents me from creating two separate transactions that are spending outputs 1st and 2nd (and specifying corresponding Key Images, as usual), both of them are specifying the 3rd output as a "fake input" despite the fact I know all secret info, but the amount of output Monero in both of them, let's say, 90 Monero and 11 Monero are used for "balancing" the entire amount of outputs.
1st (1 Monero) + 3rd (100 Monero) = 4th_Mine (90 Monero) + range proof balancing (11 Monero)
2nd (1 Monero) + 3rd (100 Monero) = 5th_Mine (90 Monero) + range proof balancing (11 Monero)
In these two cases, I spent two outputs that originally had 1 Monero each but supposedly succeeded in creating a total output of 180 Monero.