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.