Like it says in the title - on a job advert, does it usually mean that previous applicants will be automatically considered, or that they have already been considered and found unsuitable?
It means - if you have applied in the past, please do not apply again.
Normally this would mean that as one was not found suitable in the past, the company believes they will not be found suitable in the present. They are trying to save time and effort.
It doesn't make much sense that previous (unsuccessful) applicants would be automatically considered - after all, they were unsuccessful (why do I say that? Well... if they got the job, they would have been successful).
Personally, I find this kind of phrasing to be short sighted - people change and learn. If one had acquired new skills or experience that would make them more suitable to this job at this time, they should be fine to apply again, regardless of this phrase.