I'm currently reading "Eats, Shoots, and Leaves" and she addresses this .
1. If it is a modern person/modern name then you add an additional 's - "Chris's car is in the shop being repaired" "Mrs. Jones's daughter is in my class this years"
2. If it is an ancient name (Moses, Jesus) then you only add an apostrphe and no additional s - "Jesus' twelve disciplies" or "Moses' angry outburst at the Egyptian"
HTH! (and I sincerely hope I'm remembering it right...)
Mom of 4: Boy (10), Girl (7), Boy (4), Girl (2)