very serious error aid in the table

[marcofa]

happens that a piece shaped with thickness 12, when put on the table becomes 24.
this happens only for a part of a larger set.
How is it possible?
the part comes from a "new from" of equal part that however does not make such error.
I attach the r20 part file
Thanks for urgent help.
Mar

 

[marcofa]

The ark is discovered.
attention to dragging with the mouse when manipulating
evidently for a wrong movement of the mause with the crushed dx key without even the central button being crushed, a prism was dragged, and dupplicated into the logical tree, without appearing in the geometry as superimposed. but at the table the thickness of that piece is doubled
Mar