Did I 302 Hijack Myself?

[JohnKelly]

For a long time I had a redirect to my home page for any "page not found errors". I generally did this using an .htaccess file and putting "ErrorDocument 404 /index.html" in there.

While search Google to see if my domain was 302 hijacked, I noticed many old pages that has a cache data from early 2004. I have since used .htaccess files pointing to a special page for files not found:
ErrorDocument ErrorDocument 404 /404.html

I'm wondering if having so many of these old pages pointing to my home page are causing duplicate content penalties, similar to the 302 page hijack issue?

What can I do about it, and is there a better way to redirect 404 errors?