For applicants who have done only one medical test, then it is valid for 12 months from the date of their medical test.
If people have done a re-medical, then it would be bizzare and completely illogical to say "1 year from the date of 1st medical" because it doesn't make sense. Sounds ridiculous. What you have said would make the re-medical pointless!
So, going by what you have said, if the visa is issued after the first medical expires (and when the second medical has been done), then what happens? From what you have said, that applicant's visa will ALREADY HAVE BECOME INVALID and he cannot land in Canada because his first medical has already expired.
Could you please clarify this?