I'm afraid of getting nailed with duplicate content.
In general I think google just ranks one over the other, so I wouldn't worry too much about it.
That happened to me for a long time with a mod_rewritten postnuke CMS based site. We had the block on the right with links to "older articles" and completely forgot to mod_rewrite that (d'oh) so ended up with both in the index.
We were still #1 for those terms, google just ranked (what looked to me like) the first one it had indexed.
If you've now got thousands of pages indexed like that, it might be worth setting up a 301 redirect rule to point google and users to the mod_rewritten versions.