I think the name needs two M
You can go with either single or double. Both are correct.
No. I have taught several Mohameds, and all spelled their names with just one M
You can't say NO straight away. I always put 2 "m"s and duolingo accepts it every time.
There is no "correct way" of transcribing from arabic alphabet to the latin one. Here in Brazil there are a few Mohameds with one M.