[Edi]

EDIT: Academy Underneath is unique. It's an uncommon site, so it will appear in most games, especially since the number of possible underwater sites is much lower than the number of sites that can appear on land.

This much can be said of ALL unique sites: They can have any frequency, meaning that there are unique sites that are common. In most games of any size, especially if the map is large and site frequency is high, they will be found. If you found several of the same site, it was not unique.

The rare uniques are another matter, because rare sites only appear if the reroll for site determination also comes up with a rare site (all rare sites are rerolled on first occurrence).

Note that unique sites may appear more than once if they were scripted in the map file. The reason is that sites are randomly assigned first and only after that the map file scripting is added.