Why does Viridian keep saying "...oh, I've already found this"?

According to the Wiki, you may have discovered a terminal that gave you a location on a previously unexplored area of your map. Since you've already found that area, or have already visited that terminal, you are getting that message.

Also everyone needs to play this game, it's great.

The terminals are research notes left by the vanished civilisation describing their dimension. In particular, novel locations elsewhere. If you haven't yet been there, the terminal will reveal it on the map. However, if you have been there already, Viridian will dismiss the information "I've already found [that location]".

Terminal: Our breakthrough was the creation of the inversion plane.

Viridian: Oh, those bouncy lines in the laboratory? Been there, done that, got the trinket.