How to attach the same file in different posts?

When i try to attach a file in a post that i've already posted in another thread i get a error message saying i cant attach that file because its in another post of mine. How do i attach a file that i've attached in previous post?

I'd get the attachement number of the original attachment, and just link to it in the new thread.

What's the old thread?

You just use the attach bb code.

[attach]ATTACHMENT ID#[/attach]