The lists are separated because when you have 100s or 1000s of current categories the current-list becomes paginated (there are actually a lot of pages with this many categories on this site alone). If you have the remove-list merged into it, you can click to remove a category accidentally or just forget which one you selected when you change the current-page, and then perform a removal you did not intend. Or you could reach the maximum number of removals permitted in one action, want to reprioritize, and/or not be able to find where the last item you wanted to remove is. Having a summary of removals provides safeguards against this.
The most times that a category should ever be listed is twice, once in current and once in remove, so your experience of three times sounds like a bug.
But the X is supposed to change to a + in the list, and vice verse, so that you can toggle your choices from either list. Having X+ at the same time seems like a bug.