Use um loop while para manter a questão em execução e sair dela quando feito conforme mostrado abaixo
while :; do # : evaluates to true, so "while true; do ...; done"
echo "Choose A or B"
read answer
case "$answer" in
[Aa]) # match case-insensitive A
echo "A was chosen"
break # leave the while-loop
;;
[Bb]) # match case-insensitive B
echo "B was chosen"
break # leave the while-loop
;;
*)
echo "Please choose A or B only"
;;
esac
done