SEO : why pages can be available with and without URL rewrite

Hi,
All the website i made with MODX are available with and without URL rewrite.
Is this should be a problem for SEO ?
Don't you think this will be consider as a duplicate content ?
What can we do to resolved it and to force to rewrite all pages ?

Try Bobray's Canonical addon http://modx.com/extras/package/canonical

Thanks for reply @Paulp but i think you don't answer my question.

1. This extra will not force to rewrite the URL like www.website.tld?index.php?id=1 to www.website.tld
2. This extra will be execute only for Symlinks and what about Resources ? ID one in my example is a resource.
3. This can be done with a simply short code inserted in the Head like this :

If you want to execute the code only on Symlink you just have to test the Class Key : class_key
- modDocument (for normal Resources)
- modSymLink
- modWebLink
- modStaticResource
...

So my question is : Is it possible to force the URL rewrite if someone try to access page like this ?
www.website.tld?index.php?id=1