Sounds like your hop is messed up. When you try to fire the first BB, does the sound made sound different? Weak? Do you hear any air escaping? The first BB may be getting jammed in the hop, and the second one clears the jam and fires both. Try using a different brand and/or weight of BB. When I had a similar problem It turned out my VSR didn't like Geoff .43gs. I had to drop down to .40g BBs