I think as daivemac says - if you see a girl subsequent to a positive review and there's nothing major new to report, I think a simple add on to the original review - e.g. "saw her again, and still a positive, more friendly with each visit".
If the experience changes considerably and / or enough to lead to a different rating, then I think a new review helps members, particularly if you look at a girls UKP Service Providers page.
All common sense really.
I think some new members may innocently post a second review of the same girl, thinking that each visit needs to be reviewed (similar to how people do on AW).