Help with changing name with automator!

Here is what i am trying to do:

1. Make a archive of the desctop and place it in documents.

2. Change the name to the date of the creation.(of the file)

This is the ressult: Backup-01052011.zip

It all works fine, BUT if i run the program a second time the name will

be like this: Backup-01.zip

Third time: Backup-01052011

Fourth time: Backup-02

Why is this? And how can i fix it?