You can relax.. IT is not going to do any harm.. second dose of MMR can be repeated after a gap of 1 month.
But in your case it might be third dose; which will be considered null and void. We do not count it.
Your child has recieved one extra dose of MMR. (1st must have been given around 9months, 2nd 15 months and 3 rd 17 months).
Do not worry , this vaccine is quit safe. Govt has given measle vaccine ( one of the component of MMR) to many children during school, campaign in Uttar Pradesh, India (irrespective of whether child, has taken it before or not).
Relax.. If you want to read in detail about vaccines; copy and paste the below link in browser and read on.
http://www.immunize.org/askexperts/experts_mmr.asp#scheduling
Next Steps
since you are posting from London (as per your profile), your can read few FAQ from NHS, UK regarding MMR, in the below link.