I would double check the disconnector and spring are installed properly. I'd bet the disconnector isn't catching the hammer properly, allowing a slam-fire. It's most likely the spring installed upside-down.
I recommend adding a match sear and flat trigger.
https://www.robertrtg.com/store/pc/MATCH-SEMI-AUTO-SEAR-SPRING-FOR-HK-136p10.htm
Beyond than that, you'd need a knowledgeable Gunsmith to do trigger work. Bill Springfield is highly recommended...
Check out Alpha Mods. They specialize in Sigs more but they do all types of firearms.
https://www.becomealphas.com/
https://www.facebook.com/alphamods10/
alphamods10@yahoo.com