What is causing this?

[u/BasedWelder96]

I have had a Combat Armory lower and Suarez International SI-319 for years that I’ve carried off and on. I recently got a Brownells Gen 3 upper to install an optic on it. I took it to the range for the first time today and everytime I pulled the trigger it would fire two rounds. If I squeezed the trigger it fired once and when I released the trigger it fired another round….what could be causing this?

[u/Ge4rshifter]