Permalink post_id dupe content problem

I’m using the custom permalink structure: /%year%/%category%/%postname%/%post_id% on my blog.

I’ve just noticed that with my URLs, if you remove the post_id from the url the page resolves (i.e. I can access both mysite.com/2011/cat/blah/1 and mysite.com/2011/cat/blah ). This never used to be the case – when I originally set this up if you removed the post_id from the URL, it would forward you to the URL with the post_id included. Older posts have this redirection working fine; newer posts don’t appear to have the redirect working.

Does anyone know why/how this has broken & can help me fix it? I have over 3,000 posts on my blog and I’m not sure when the problem started. Aside from updating software, I don’t think I’ve made any changes to the site so I’m not sure why this is now happening.

Thanks in advance!