Friend has a Colt AR15. Came to me with the complaint that very often he is getting 2 shots with every pull of the trigger. Although he has had others try firing and it does not happen, only when he fires. I have examined and fired without any problem. Then he fired right along side of me and it double fired. Watched while he fired and it only happens to him. Somehow he seems to be riding the seer with his trigger control. Has anyone else experienced or heard of this problem?

The simplest solution is to replace the fire control group: Hammer, trigger & disconnect. New springs wouldn't hurt either.

aware of the solution,  but don't understand why it only occurs when this individual shoots the weapon.  Can his trigger control be riding the sear??

Has anything been done to the fire control group?  A lot of times these issues crop up after someone fiddles around with it.  

First check/replace the disconnector spring (make sure the large end is down), then disconnector, then hammer, then trigger.