That is, if the child has previously received immunization MMR (Mumps, Measles, Rubella), still must get the MR vaccine.
MR is given again in lieu of a second administration of MMR, because MR is cheaper and protection against Measles and Rubella are very important.
Its not given in addition to, it's actually replacing a second administration of MMR.
In countries where two doses of MMR are given, then NO MR vaccine is administered as it is redundant.
RE: One MR Vaccine Prevent Two Diseases