On my individual dynamic entry archives I get this error at the bottom of every entry:
fatal error: cannot redeclare is_valid_email() (previously declared in [I've removed the path information for privacy]n/mt/php/mt.php:745) in [I've removed the path information for privacy]/mt/php/mt.php on line 751

Any idea what could cause this? I haven't found anything in my searches. I've also been generating "core.###" files in my MT directory lately and filling up my webspace. Related?

EDIT:
Even when I totally clear the template blank, I still get the error on a totally blank page. I've also compared the mt.php file to one from my other site that works fine and they are identical so it's not the file. It also appears to happen only on one of my blogs and not the other.

what plugins do have installed (enabled and disabled)?

I had the exactly same problem on MT4.21. Disabling "Enable conditional retrieval" under the publishing settings of the blog solved it. Hope someone is reading at the HQ.

Best thing to do is raise it as a bug http://www.movabletype.org/feedback.html