If you need to find 3D faces (3DFACE) with duplicate vertices in your DWG drawing - i.e. faces with overlapping vertex points 1/2/3 - you can use the LISP utility Sel3Face and its command SEL0FACE.
Download Sel3Face from www.cadstudio.cz/download, load the .LSP file with APPLOAD and run the command SEL0FACE. From the selected 3Dfaces, it leaves only those where it finds degenerated vertices - e.g. with equal first and third vertex (3dface degenerates to a line). The faces will stay grip-selected (e.g. for Delete), or you can use this command on the prompt "Select objects:" inside another command.
Similarly, the command SEL3FACE selects 3D faces without the fourth vertex, i.e. trilateral 3DFACEs.
You can set the tolerance for matching vertices using the FUZZ variable (setq).
The newer and more advanced utility SelFace (see www.cadstudio.cz/download) can conveniently select also singular faces (1 vertex), horizontal faces or faces with zero elevation.
- Trilateral - faces with three vertices, resp. with the 4th vertex = 3rd vertex
- Quadrilateral - faces with four distinct vertices
- Degenerate - faces with 2 matching vertices (from 3; =line)
- Singular - faces with all matching vertices (=point)
- Horizontal - horizontal level faces (same elevation of vertices)
- Zerolevel - horizontal faces in Z=0
- Cclockwise - faces with counter-clockwise vertices