The "repeat visitor" function is set at the script level, and only works with IE tests. It specifies that the script is executed once as recorded (caching the content), and then re-executed with the content still cached. Only the data from the second, cached run is reported in the Synthetic Classic Portal.
A "repeat visitor" IE test is like any other test, but should be presumed to run more quickly (i.e. have a faster response time) than the same test run without the this function turned on, because it uses cached content.