Nothing. Men do not have symptoms when a woman is pregnant.
Yes. Such cases are reported. This condition is called as superfecundation.
No
Only the first sperm influences what happens so it is physically impossible.
Men can not get pregnant - neither individually nor in pairs.
If the vasectomy was successful then no the woman cannot become pregnant.
Its been recorded that a woman did indeed get pregnant while she was already pregnant, but I think it was due to the fact that she had two uteruses.
Yes.
yes he make two woman pregnant...
A man is never too old to get a woman pregnant. Men , unlike women , are fertile from puberty until death.
Yes, provididing there areno other problems
Not until she starts to show (or until she tells him).