How is done that?
When is search the links for some site (domain.com) i found 100 for example but some time many on this 100 links are from the same site (domain.com)

!!!
where is the reason for that ?

It probably has something to do with subdomains. WWW.domain.com is NOT the same as domain.com.

Well a link from your contact page to your home page is still a backlink for your home page. Backlinks are measure for the page and not the site overall.

For example as you post more here most of the threads you post in will show as a backlink to whatever page is linked in your signature.

All those links won't carry the same weight. The first one or first few from the same site will carry a certain weight and the rest will more than likely lose weight quickly, but all might still show up as backlinks.