Very rarely (only had it once), if a Duplicant has 2 equally fast paths to the next target, they will cycle back and forth between them deciding to go the other path.
When this happened, I had speed at 3 and they were at a 1 tile ledge climbing up and dropping down over and over. It's possible the timing was such that their Athletics combined with the length of the ledge climb/drop animation and speed 3 was matched perfectly with how long it takes to repath.
Possible solution: Whenever working out a fastest path, assume they are at the point their current animation will take them to. Alternatively, only accept a new path if it shaves off more than a certain distance.
Unsure, seemed more like a fluke than anything else.