Found the same (BMVert) used multiple times I'm trying to import the following .obj file into Blender

v -1.3433 0.3333 -0.7191
v -1.4807 0.3333 -0.3596
v -1.2747 0.0000 -0.8989
v -1.5493 0.0000 -0.1798
v -1.3433 -0.3333 -0.7191
v -1.4807 -0.3333 -0.3596
f 1 2 4 3 5 6


And getting the error specified above. When I switch the order of verts, it seems to be working. Any idea what causes this error and how can I avoid it?
seems to me that's a malformed polygon: