Stick to your guns its a double tap and wrong, I have written that up many times with no debates, and electricians fixed them. When people get lazy things are not done correctly and then they just hope they never have to answer for the laziness. I once went back and forth with a homeowner who was a master electrician at the Meadowlands on the 12 gauge wire suppling his 4500 watt water heater.
If you get away from the term “double-tap,” it may make your position more clear. The electrician may be thinking “tap-conductors,” in which case he is correct. Address the “terminal not being listed for multiple conductors,” as that is the real issue.
Update…master electrician repairs the panel properly.
I really do trhink Jeff hit the nail on the head. Terminal not listed for multiple conductors has now take the place of the phrase “double tap”. He is correct, the better we explain it in their ‘terms’ the better they understand it.
Chalk one up for the good guys…
Post 3… 
I’m glad the “master” repaired as needed.
NEC 110.3(B) requires that electrical equipment be installed and used in accordance with its listing and labeling. Even though the electrician is correct regarding the tap conductor rule, he is overlooking the incorrect use of the lug. A lug for two conductors will have separate connections for each conductor. Always look at the panel manufacturer’s installation instructions posted on the panel. These give the range of conductor sizes and number of conductors allowed for the panel connections. Note that NEC 110.3(B) has been in the NEC for many years.
Not always, take a look at the lug in the center of the photo in post#18. 
It does not matter how you explained. When the electricain saw it, he should have know to correct it. They are suppose to know more about electrial than us simple minded home inspectors. :roll:;-):mrgreen:.