To me an icon is meant to be double clicked (due to heavy windows use I would guess). If you want something to indicate action in a single click you should be using a button. The key features of a button are that it appears raised and depresses or makes a sound when you click it each time.
If the users can see or hear the button action on the first click they will be far less likely to double click in the future.
PS. I've observed many users who either don't understand the difference between single and double-click, or don't have conscious control over whether they do one or the other. So, as a general design principle I would say to avoid double-clicks completely in your own designs.
If using a number is not an option, then try to use ripples (circles) to represent the number of clicks; 1 small circle inside 1 larger circle to represent a double click and 1 large circle to represent a single click.
I think the issue here is that single and double click icons will only work when next to each other as it adds the necessary meaning.