Installation problem Rated 2 out of 5 stars

Can't install, get this message:

Firefox could not install the search plugin from "https://addons.mozilla.org/en-US/firefox/downloads/latest/12142/addon-12142-latest.xml?src=addondetail" because an engine with the same name already exists.

why?

This review is for a previous version of the add-on (20090608).