Receiving error 'The blog already exists'

The clones plugin has been installed and configured however when attempting to clone an existing site, any new slug that is entered returns the message 'The blog already exists'.

The new site wants to be cloned onto the slug 828 and there is no site already on this, neither active or de-activated. There is however a site on 1828 which I suspect to be causing the confusion. I have also attempted to clone the site onto 0828 but this receives the same error.

I appreciate your assistance with this.