Nearly every brass player will encounter the double buzz at least once in their lifetime. It essentially means that you are not taking care of your chops in the best way (no offence meant)
Think of it as a “growing pain” of sorts.. you are getting better, which is great, but not all aspects of your playing will improve at the same rate. Without knowing your playing its impossible to tell you what area the trumpet you are neglecting or overdoing. Its great that you are studying with a good teacher, he will definitely be able to help you with this. Until then just try to make sure to get a good warm-up all the time (easier said than done, I know..)
Technically I believe the double buzz occurs when a tiny piece of the lower lip stops vibrating or vibrates half as fast as it should (seems like that would be impossible right??) This makes the dreaded ‘rasp’ sound, usually down exactly one octave. I have heard top professionals struggle with this as well as a few of my junior high students. I got it a couple times in high school.
Here is an excellent link
http://mailer.fsu.edu/~bgoff/tpt-tips/tip107.htm
good luck.