Why 2 MMR's
Actually the 2nd MMR
is not given as a booster like all other vaccine series but because 15% of first MMR recipients are 'non responders' and the second dose is meant to catch those in that 15% group.
Two MMR doses were found to catch most everyone where one dose missed. If you, instead, have serologic proof you responded to the first dose, repeat vaccine is not needed.