I am creating a figure with armatures and IK constraints. When I go into pose mode, the right lower leg gets duplicated; the other is fine. I checked for duplicate objects by 'merging vertices by distance' that returned 0 objects found. Next, I followed the instruction on similar questions by re-assigning the parenting of the body to the armature, but that didn't help either. Next, I selected view by bounds, as explained in another answer, which also did not solve the problem—attaching the .blend file through Blend-Exchange.
A section of object duplicated in pose mode. [Answers provided in similar questions did not help]
You model has huge problems with face orientation:
That's why it cannot work.
you can check face orientation in viewport overlays -> face orientation.
Red means: wrong, blue means: ok
So select your object, edit mode -> Shift N (recalculate normals)
Reassign body to armature with automatic weights, then it works: