I sometimes have an icon I use for 2-3 distinct purposes. If I later upload a better icon for one of those purposes, I take the keyword off the one icon (which I retain for its other purposes) and put it on the new icon. Other times I might merge two icons that see less use than I thought they would and that can both fit with one visual image, but the concepts behind why I use them or when I use them may be distinct enough that I don't want to merge the keywords or the uses on other posts.