Tire Plugs are legal and when installed correctly they are just as safe as a tire patch.
Tire Plugs that are installed correctly should last the life of the tire.
Tires plugs will last the life of the tire when the tire plug is properly installed.
A Tire Plug that is installed correctly is a permanent repair although tire patches are much better.
Tires can be plugged more than once as long as the holes are not too close together.
You can plug a tire twice if you get a hole in the tire more than once.
However patching a tire is much better but when done properly a good tire plug can hold and last until the tire needs replaced.
I've plugged tires 4 times before and the tire lasted until it needed replaced at 50,000 miles.
Just never put a tire plug in a sidewall as that is not safe.
if the tire has a hole in the sidewall it should always be replaced with a new tire instead of trying to plug the sidewall.
I have however had to in an emergency plug the sidewall of my own truck tire because I had no spare with me and was on the side of the highway and needed to get off the highway.
But I did go get a new tire installed within that same day.