There are some instances where a polygon may have vertices that are stacked exactly on top of each other. This can lead to a number of issues in exporting to CNC machines, creating extrudes or other solid operations based on polygons with stacked vertices.
(Appears to be a single corner vertex)
(Moving it to the left reveals the second vertex beneath the first)
To resolve this, select the polygon or polyline and use Modify > Drafting Aids > Simplify Polys, in the first mode, Maximum Deviation, you can leave the setting at 0 and running the Simplify Polys operation will remove any perfectly stacked vertices without affecting the overall shape of the poly.
Edited by JimW