tell me one thing When u are optimizing your website ..then how Could u stop those Back links which is linking to you and Specially from The Same Domain
because Multiple backlinks from the same site is not considered very useful .it increase only No of Back links And Not Not Quality Links
Yes you are right using same sites for backlinks is not a good practice in SEO. Always try to promote site in different sources which has good PR.
it works like a vote, when you get one backlinks from a particular website then you will not get any benefit for building another backlinks on the same website.
Internal link may not have much weights compare to link from external website but it doesn't hurt having it. A good internal link flow would help more indexing pages and better for inner pages ranking. If you link between only relevant pages then it could be a big plus.
You can use "nofollow" to shape your sites internal PR. For example if you have a link to "privacy policy" on every page but don't want it to rank extra well then use nofollow on those links.
yes, but you have to build links from various sites, which are relevant to your site, so that only you can get better ranking in google