Url permalink slug bug

Weird one this…

So on the edit page for a page, you can see what the url is going to be. This is normally some version of the title of the page. On making a page entitled ‘Making Anatomy Slides’ the url slug decided it wanted to be “making-anatomy-slides-3”. I’ve tried and tried and I can not get rid of the ‘-3’.
– there are no other pages called that on the site,
– if I change the hyphens for underscores then the ‘-3’ isn’t there,
– if I try removing the ‘s’ from the end the ‘-3’ goes
– if I call it ‘anatomy-slides’ the ‘-3’ goes
– on the pages index the ‘-3’ isn’t shown if I try to quick edit – but its back again if I try to publish
– if I publish the page then go to the page without the ‘-3’ it redirects the page with the ‘-3’
– if I go to ‘-1’ or ‘-2’ urls those both 404.

What is going on, does anyone know?

I would publish the page for people to see but it is a public site and the document is not ready for publication yet.