Merck used to proudly state on its website that one MMR vaccine provided lifetime immunity to 95% of those who received it.
Which makes one wonder why 100% have to have a second MMR. There are no studies that I could find that showed how many of the remaining 5% developed immunity from the second shot. I know quite a few people (adults and children) whose doctors insisted on giving them SEVERAL repeat doses of MMR, of varivax, and others, because tests showed they weren't immune to those diseases.
In spite of thousands of reports to VAERS of severe MMR reactions (including many who WERE compensated in US vaccine court), the CDC does not suggest a titer--a blood test to determine immunity.
I would like to make sure everyone reading here learns that a positive titer is accepted in every state in lieu of a vaccine.
If your child got an MMR as an infant, you may ask your doctor for the blood work to be done, or you may choose to do it on your own and pay out of pocket, and probably avoid the second MMR.
You also have the option of delaying the first one until your child is ready to enter school.
I would suggest, however, that you not combine it with any other vaccine, as vaccines in combination have never been properly tested for safety.
For adults, if you got the MMR as a child (or had measles), and your doctor suggests that you need another one (because the immunity resulting from MMR, like all vaccines, is proving to be much shorter-lived than originally anticipated), you can likewise request a titer.