two different people having the same date of birth or same day as
birthday (mind it, only the date is same, because iso = same, dob = date of birth; year may be different)
like isotopes, which are two different atoms (with different atomic mass number) having the same atomic number.
like
Mayank & Paremal having their
birthday on the same day, call each other:
Paremal says to
Mayank, "Hey Isodob, wish you happy
birthday!"
Mayank replied, "Thank you and same to you my Isodob!"