I get that on my mac at home sometimes too...while it's not a great fix, I did find that clicking on the Remember the Milk header would refresh the list and often remove old or duplicate tasks. Does that work for you too?

Hi remember_festival,

I'm not sure if the developer of the widget checks these forums (it was created by a third-party developer), so it might be worth contacting the developer directly to report this issue.

Hope this helps!