BEGIN { id = 0 }
$1 == "lastName:" { lastName[id] = substr($0, index($0, $2)) }
$1 == "firstName:" { firstName[id] = substr($0, index($0, $2)) }
/^$/ { id++ } # empty line
END { for (i = 0; i <= id; i++) {
          if (lastName[i] != "") {
              printf("%s", lastName[i])
              if (firstName[i] != "") printf(", ");
          }
          printf("%s\n", firstName[i])
      }
    }