This usually happens when the AutoCAD drafter uses construction objects like lines, arcs, and polylines to help generate Solid Hatch boundaries, but then forgets to remove those construction objects (or place them onto a different layer). AutoCAD doesn't do a good job of displaying overlapping data so those construction objects usually remain hidden and become forgotten by the AutoCAD drafter.
Then when FAB 3000 converts the DXF both the Solid Hatch and the construction objects are converted resulting in duplicate data (ie. 2 boundaries). Duplicated data is never recommended and usually causes some type of difficulties.
Fortunately FAB 3000 has a DXF/DWG Import option "Ignore Hatch", which can control what objects from the DXF are imported.
Unchecked: Hatches will be processed.
Checked: Hatches will be omitted. This will cause FAB 3000 to ignore the overlapping solid hatch, and only convert the LWPolyline.