I'm not sure how this could happen but I saw it happen on our other seat of Solid Edge. Our other engineer had an assembly open and when he would uncheck the visibility of the part in the tree, SE would add another occurence. I've never seen this before and I'm not sure if I could repeat it but it did happen - very strange.
What version of Solid Edge is the user using? This sounds like the same issue discussed in various other threads where selecting an occurrence in the PathFinder would inadvertently place another occurrence.