What is a unique visitor? A unique visitor at StatCounter is based purely on a cookie. You can specify your visitor session length from 30 mins up to one week. Basically the first time someone visits your website they are considered a unique visitor. Then a tiny cookie with no personally identifiable information (just your project id and a simple variable) is created.
If that visitor session stays active, by them visiting your website again and again before the session time expires then they won't be counted as a unique visitor again. But if the session time has expired, they are counted as a unique visitor again and a new session is created. And it starts all over again.