I wanted to ask if writing the same post which i prepared one time and pasted it in many sections in the forum would decrease back link as i saw that google indexed only one topic of them and considered the others in the omitted results included are those omitted results are counted or not ?and what is the way in that i would prevent that ? can i change the title of the post in sections only or should i change words of the post in each time i put it in new section or the two?
I don't know what forum you're referring to, but any one I've seen would regard copying the same post as spamming and delete.
Yes be careful what you do. A lot of people sign up here, post the same spammy message in lots of categories and also pm it to a lot of people. The result: a BAN
To answer your original point, in any forum, in any website, duplicated content whether internally or externally duplicated can often not be indexed by Google.
Don't do it here and in other forms. You will banned for spamming and probably your account will be deleted. I don't know DP rules much but in other forums they are deleting such cheats. And be ethic,it is unfair.