I can’t see there ever being a proper situation where you would export a part and then import the same part. If you just exported the SVG you want to edit and then import the edited SVG this would never happen. Again this is a situation where trying to not use the editor provided to you to edit the files causes more problems.
Also for Fritzing to recognize changes in the parts bins you must restart Fritzing and it will ask if you want to save the changes. I am not sure it this solves the issue created by reimporting an existing part that you have just deleted but it may. But again if you didn’t create the problem by trying to circumvent the editors function you wouldn’t need a work around. If I want to replace a part I will just change the SVGs and metadata of that part or create a new part and then delete the old.
Sorry if I sound negative, that is not my intent and everyone should continue doing things the way they are most comfortable as that is what I advocate. People should be able to work the way they are most productive rather than be forced to do things as someone else says it should be done.