Yes, I'd suggest visiting your GP to get a second dose, if you are unsure whether you've had 1 or 2 shots. Your doctor can do a simple blood test to check if you have immunity. However, the blood test can actually cost more than getting the second shot of the vaccine, so if you're really worried, just go ahead with the second shot!
2 doses of the MMR vaccine grant life-long protection against measles, even into adulthood. One dose of vaccine is 95% protective, and two doses provide 99% protection. This is because between 2% and 5% of people do not develop measles immunity after the first dose of vaccine. The second dose is not a booster, but rather produces immunity in the small number of people who fail to respond to the first dose.
FYI, under the Infectious Diseases Act in Singapore, it's mandatory for children to undergo two doses of the combined measles, mumps, and rubella (MMR) vaccine during their lifetime - the first dose at 12 months of age and a second dose at 15 to 18 months.
