Some explaining comments:
Any element that has at least 1 event listener added to it via 
Event.observe() has the attribute '_prototypeEventID' set.  This 
attribute links to the location in the Event.cache() which contains 
the observe information.  This code snippet will return the 
Event.cache element if it exists and if it has at least 1 observed 
event (looks like the Event.cache array is not pruned when the last 
observer is removed)

I have implemented a javascript access key handler in CVS HEAD which 
seems to work great so far. It doesn't handle onclick events yet that 
have been attached programatically though.

Here's some code snippet from Michael that should do that trick:

$H(Event.cache[$(element)._prototypeEventID.first()]).findAll(function(h) { 
return h.key && h.value.flatten().size(); });

Brilliant on the Greasemonkey script. Works perfectly. Thanks!

I wrote a Greasemonkey user script to remove duplicate accesskey 
attributes to deal with this issue.  People can use it until IMP is 
fixed.

http://userscripts.org/scripts/show/29539

Yes this is a very large issue that was overlooked, many online forums 
(vBulletin) have an issue with this double access key issue. I need a 
solution ASAP! if anyone can come up with it.

I thought so too, but at least in IMP, since we re-use the same 
template to render the bottom navbars, we still get doubled accesskeys.

This is already the default behavior of the access keys methods.

In Firefox 3, if we output two identical accesskey="..." links on a 
page, then pressing the accesskey will select the first matching link 
instead of following it. This happens on pages where we have top and 
bottom navbars, for instance.

We should figure out a way to either never output a second instance of 
the same accesskey, or have some javascript that post-processes links 
and removes duplicates.