A bit more of a detailed answer for you -
How Mamoswine's first attack works is like this: You flip until you get tails, then you can do one of two things:
~Go along with the result and deal damage accordingly (30 damage times the # of heads you got).
~Start over and declare the previous series of flips void (as if they never happened). If you do this, put 2 damage counters on Mamoswine (unless this would KO Mamoswine, in which case you have to do the first option). Then, you flip until you get tails again.
It says 'start again', so it is basically doing a fresh start with the attack. In both your cases it would do 30, because on both instances you flipped one head only (they don't accumulate from previous attempts, since 'start[ing] again' declares the other flips void).