Permalinks Question

Hello,

I created a post and the permalink structure was like:

sitename.com/postname

I deleted the post then made another post with the same postname and it has a permalink like:

sitename.com/postname-2

When I try to edit it so the permalinks looks like this again:

sitename.com/postname

it reverts to this when I save it:

sitename.com/postname-2

How can I save it and remove the -2?

Hope that makes sense : )

Thanks.