Sounds like an overclocking error to me 😀
On another note that thing is a direct copy of a Sig Sauer P2022. After owning one for a little over a year I can report the Sig version does not shoot 2 rounds at once. Ever. 😉
Double feeding is actually quite common with CO2 BB guns. I have no idea how to fix it. I have not owned one since I was a kid. I am thinking about buying a higher end nitrogen piston driven pellet rifle like the RWS Diana. They hit as hard as a .22 at short range and are safer in neighborhoods.