I expected to get 'false' from the following statement:
SELECT ST_IsValid(ST_GeomFromText('POLYGON((0 0,10 0,10 10,10 10,0 10,0 0))'));
Instead it returns true! Even if the vertex in 10,10 is followed by another one with the same coordinates. Maybe I misunderstood something and/or I am making some mistake. Where can I find a complete explain of that makes a geometry not valid?