To teach is to learn twice,true or false? Why do you say so?

Yes, because it takes great effort to prepare the lesson. even if you dont learn twice you still renew your information.

This is a truth. One way to know that you have truly  learned something beyond basic application,is when you can teach someone else. Learning something is a process. One can not be assured that they have learned something until they can implement what they have learned. One cannot be assured of mastery until they can teach someone else.

 One acquires information, converts that information to knowledge, and then implements that knowledge and they can then trust that they have learned. When you can teach someone else you realize that you have cemented that knowledge and you have grown beyond the ability to simply implement what you know but you have learned to expand that knowledge. Your knowledge now takes on a whole new dimension.

Strictly speaking if you teach purely what you know then there is nothing gained. However if you research your subject more and improve your own knowledge and understanding of the subject before teaching then it's a definite YES. It's a question of interpretation.

I agree it's true

no faintest idea

perhaps thrice not just twice.Think of a topic you learn it for the first time , it may be difficult.If you learn it again , it will be less difficult.If you teach it , you need all subitems .

that's absolutely true

Yes it is true. It takes alot of effort to teach someone & more extra effort to make them undertand what you have taught them. Learning starts a the preparation stage, you have to prepare what to teach, so through this preparation you are already learning in the process. As you teach someone, you brainstorm, recall & store various facts at the same time-learning. As you continue teaching, perfection brings about grasping various facts.