Strange behavior on numeric URLs

If I try to create a post with a numeric URL, WordPress adds a “-2” to the end of the URL. How can I stop this?

It doesn’t matter what I choose … it can be a big long random string of number that I KNOW I haven not used anywhere else, such as:

http://mydomain.com/07923904209878234072987349/

And when I save the draft, it will end up as

http://mydomain.com/07923904209878234072987349-2/

This behavior goes away if I add a single letter, such as this:

http://mydomain.com/07923904209878234072987349a/

Has anyone encountered this before or know how to fix it?