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When you have to promote different pages of your one website for a certain keyword, the pages compete each other for a certain keyword within your site; the situation is called Keyword Cannibalization.

Is it a negative point in search engine ranking? If it is, how to avoid it?

Since this mod only affects forums Keyword Cannibalization isnt relevant.

But if u put the same keyword for 2 pages of my site...then one page give competition to my another page....

Yes i think keyword cannibalization can have some negative effect.

Keyword cannibalization isn't an issue that's in the SEO forums much, nor is it something that many SEO's feature prominently in site reviews, but it can be detrimental to potential rankings for several different reasons. First, I'll illustrate how keyword cannibalization happens.

Keywords aren't such an issue for Search Engines as they used to be due to keyword loading that designers started to use. SEO depends on a variety of factors, from page title, topic title, h1, h2 headings, meta description, keywords, internal links etc.

Keywords help with 'categorisation' of your content to help people find specific content, and does not really do much with page ranking.