[jQuery] checking for multiple selects with the same value

[RwL]

I'd like to extend this simple function but don't know what to do
next:

$('select').change(function(){
        $('.formTip').show();
});

What I really want this to do, is:

On the change event, check to see if any value of any <select> element
on the page matches the value of any other <select> on the page -- if
so, show .formTip. If they've made a change so that two values no
longer match, hide it again.

I'm still wrapping my brain around all the ways jQuery can select and
extend the selection; can't figure this one out.

Thanks,
Rob