http://www.wpro.who.int/china/media_centre/press_releases/pr_20100901.htm

Some may worry about their child receiving multiple doses of vaccine. However, a child who has previously received measles vaccination can be given an additional dose of measles vaccine. "The immune system can easily manage multiple or repeated vaccinations,” stated Dr O’Leary.

Vaccinating every child, even those that have been vaccinated in the past is essential in stopping the virus with a wall of immunity in the population. Using this strategy, which was developed by WHO, has enabled countries in Central and South America to eliminate measles.