Hello, at that time code allowed that. It is no longer code to do that, but you are grandfathered in. Should the panel be changed it wouldn't be allowed. I also, see it, too.
You are allowed to have more than one wire but, you won't see 15 or 20 altogether. I am thinking that is what you are seeing. Some city could require one wire per lug, but that is something the city would require and not the NEC.
I might be wrong but a double tap would be a wire connected to a part of the panel directly connected to the power pole with no protection inbetween.