One of the main objectives of SEO is to establish a pecking order through relevance, chronology, prominence and authority within each page of your website.
Mapping the focus of each page to accommodate the on page and off page reputation can dramatically increase user engagement and lower bounce rates vs. leaving your pages wide open like a free for all when it comes to scoping relevance. For example, say you have optimized a page for a specific keyword and you have 5 other pages within your site that are also targeting the same keyword.
This leaves search engines confused about the origins of which page is the true champion / preferred landing page and disperses relevance. Sure, there is nothing wrong with creating a double listing for a keyword in the search engine result pages, but what about the user? are they finding the most relevant page in your site, and if so, from what keywords?
Creating the keyword pecking order /hierarchy is essential so you eliminate pages canceling each other out like crabs in a barrel suppressing each other. As soon as one page attempts to create prominence (for a keyword), one of the other near duplicate pages latches on and pulls it back down into the barrel for the sake of normalization.
The more pages you have within a site, the more opportunities you have to rank for multiple keywords (or a specific keyword) if you understand how to map internal and external links.
Synergy is the idea, but competing with yourself should be avoided if at all possible. A few quick remedies in this situation would be:
1. Shift the flow of link weight by electing the strongest page (based on analytics data) and link to it from the subordinate pages.
2. Use the noindex, follow command in your meta tags for pages with less compelling content so that page does not compete in the search engine index, yet it passes link authority via osmosis to the target page.
3. Go back to subsequent pages and edit them to increase the frequency of alternative variations of the keyword using modifiers (adding additional words surrounding the main keyword). You could also consider emphasizing a plural version of the phrase with an alternate key phrase combination.
In either case, you dampen the effects of having multiple pages fighting for the same keyword that could trip a filter in the search engines that doesn’t allow any of the pages to reach their full potential.