Many people double up (or more) on the same numbers. The idea is that, should numbers win the smaller prizes, winnings are multiplied. For example, second place $1 million prize is not split like the jackpot, every winning ticket gets the full mil.
I don’t really get it, but I see people doing it.
Yes
The grand prize is divided equally among winners so there is no advantage but smaller prizes would be doubled, as you said