I need to extract the vertices coordinates of the four corners of each rectangle of a layer made up of 729 polygons (= map sheets). I began using Vector/Geometry Tools/Extract Vertices function to create a vertices layer but each rectangle contains 7 vertices - the first and the last are superimposed, I suppose - so I want to eliminate the median vertices and one of the two (the first or the last) vertices superimposed. Moreover the 4 vertices of each rectangle coincide with the vertices of the adjacent rectangles having the same coordinates but different attributes related to the different map sheets.
Now I do not know how to delete the unwanted vertices (I can not use the Processing / Vector General / Delete Duplicate Geometries function because it would also delete the vertices of adjacent rectangles). How to solve this problem?