If you're doing it a year early to add another subject to your list, that would look good to unis. If you're doing it early to reduce your workload next year, that would not look good because other applicants are capable of handling all subjects at once. I believe that's what Muttley is saying.
I did my GCSEs two years earlier but I did all of them at once. If I had spread them out, it wouldn't have looked good on my application.