First of all medical school is an undergraduate education no matter what country your from. Your ability to practice in all 50 states may factor on where you graduated from medical school though.
For example.. Someone who went to medical school in England may have got in right out of high school and finished just their (MbChb) medical degree, then is eligible to become ECMFG certified and apply to a US residency in all 50 states. Degrees from some of the 'carribean' schools are not able to be licensed in all 50 states, so in that case... no matter how many degrees you have, you'll never be able to attain a medical license.