Once you are pregnant, if you were to have sex with another person while still pregnant, you would not get pregnant by the second guy.
However, once the baby is born, all bets are off. You could get pregnant by someone else if you are not pregnant any longer.
I am a little confused. If you mean once the sperm has penatrated the egg then you can't be pregnant by someone else this is true but in rare cases one twin may have one father and the other twin may have a different father. But one father to one baby. If you mean you have a baby and then can't have another by someone else, no that's not true.
i don't fully understand your question, if you have intercourse with your partner and you both conceive you cannot get pregnant to that person again as you already have a small foetus growing inside of you.
If you mean after you have a child can you get pregnant by someone else then yes you can