$1 == "lastName:" { lastName = substr($0, index($0, $2)) }
$1 == "firstName:" { firstName = substr($0, index($0, $2)) }
/^$/ { emitRecord() } # empty line
function emitRecord() {
    if (lastName != "") {
        printf("%s", lastName)
        if (firstName != "") printf(", ");
    }
    printf("%s\n", firstName)
    lastName = ""; firstName = "";
}
END { emitRecord() }