[Plugin: WordPress SEO by Yoast] Problems with Permalink writing to htaccess file

I seem to be having a problem whereby the re-write rules are constantly being added to my htaccess file even though they already exist. This keep being added to the end which increases the size of the htaccess file until the site wont actually load!

Any ideas how I can fix this?

WordPress SEO is the only plug-in I am using that uses the re-write rules.

thanks

Andrew