Your laser cuts every line twice? The file really contains double lines.

Every cut runs twice: edges char, jobs take twice as long, thin engraving lines come out as double strokes. The machine is following orders. In almost every case the file genuinely contains each line twice, and it does so in one of two very different ways that need two different fixes.

Mechanism 1: true duplicate entities

CAD exports produce stacked copies more often than you would expect. Two adjacent parts share an edge, and the exporter writes that edge once per part. A Fusion 360 or FreeCAD sketch contains projected geometry, and the projection goes out along with the original. The result is two coincident LINE or polyline entities, and the laser dutifully cuts entity one, then entity two, in exactly the same place. Because the copies overlap perfectly, the drawing looks completely normal at every zoom level. That invisibility is the defining trait of this mechanism.

Mechanism 2: the outline of a stroke

A line drawn in a graphics program has a stroke width. Auto-trace (raster to vector) and stroke-to-path conversions preserve what the line looks like rather than what it is: each stroke becomes a thin closed outline, two nearly parallel lines connected by small caps at the ends. The laser cuts both long sides, a fraction of a millimetre apart. At working zoom this outline is indistinguishable from a single line, which is why traced logos and scanned drawings are the classic source of "everything cuts twice".

Which one do you have?

LightBurn gives you a test that doubles as the fix. Select everything, ungroup, and run Edit > Delete Duplicates (Alt+D). The function removes only objects that are identical in size, shape and position, which is precisely the definition of mechanism 1. If the doubled cutting disappears, you had true duplicates and you are done. If nothing changes, zoom far into one of the lines: two parallel paths joined at the ends means you are looking at stroke outlines, and no duplicate remover will touch them because nothing is duplicated.

Fixing true duplicates

LightBurn

Edit > Delete Duplicates, shortcut Alt+D. Ungroup first, because the comparison works on individual objects, and remember its limit: only exact copies in the exact same spot are removed. Two lines that overlap along part of their length survive. Details are in the LightBurn documentation.

AutoCAD

The OVERKILL command deletes duplicate and overlapping geometry and can merge partially overlapping segments, which makes it the more thorough cleanup if the file passes through AutoCAD anyway.

RDWorks

RDWorks ships a built-in function for deleting overlapping lines, so Ruida-based machines can clean the job right before cutting.

Fixing stroke outlines

There is no one-click repair for mechanism 2 in any mainstream tool. The realistic options: go back to the vector source and export the actual lines instead of a trace of them, avoid converting strokes to paths before export, or node-edit the outlines down to single paths by hand. That is tedious, which is the honest argument for prevention. If the artwork only exists as a raster image, a trace will always produce outlines, because the tracer cannot know the image was ever made of lines.