There is no reliable event fired when a selected option is re-chosen.
Whilst on some browsers you can catch events originating from an <option>
(which you could use to trap click and keyboard events if you had the patience to try to reproduce a browser's select handling), this is unstandardised and doesn't work in IE (as it uses native Windows widgets to implement select boxes, which don't give that kind of granular access).
If you need to be able to re-fire an event when the same option is chosen again, what you have doesn't really sound like a select box to me; it could perhaps be better replaced with a scripted pop-up div full of buttons. (There are plenty of scripts that will substitute a select box for a scripted div to give you greater control on browsers where JavaScript is available.)