Say i have 5 stages for my case. A-B-C-D-E. Case is currently in Stage D and i have used pxChangeStage flow action or pxChangeStage utility to jump to stage B by selecting 'Stage B' in the flow action inputs.
It is actually going back to stage B and also going forward to stage E as well. Said that, it is creating two parallel assignments one for the step in stage B and for the step in Stage E.
In the Change Stage Smart Shape,you can have an option "Close current Stage Assignment on change stage",If you select that option ,the current active assignment will be closed and only the change stage assignment will remain open.