Blog tags creating multiple pages, how to fix?

I have a blog tag of "Payment Disuputes".

This generates the page www.website.com/blog/tags/Payment+Disputes but for some reason it's also creating the page www.website.com/blog/tags/Payment+Dispute, these pages are 100% identical and are causing duplicate content page errors. "Payment Disputes" is the only tag added to these blog posts so I'm not sure why it's creating the 2nd page. Any ideas?

did you have one somewhere at some point without the s?

check the database and get rid of it? then try again?