One of the most consistent problems I see when conducting site reviews for clients is keyword self-cannibalization - the practice of heavily targeting the same keyword phrase on multiple pages of a site. In my opinion, sites are shooting themselves in the foot when they do this. Let's look at a few quick examples:

SeattleTeamRealEstate.com

It's pretty clear that they want to rank well for terms like "Seattle Real Estate" and "Seattle Real Estate Agent." In fact, they're trying so hard, they've got those exact phrases in the title tags of dozens of pages.

BarnesandNoble.com

It's not just smaller sites with this problem. I note that a search for Ferris Bueller DVD brings up BarnesandNoble.com in the 28th and 29th position, and I'm guessing one of the reasons why is that they've split up their link pop, anchor text, and targeting between multiple pages targeting the same traffic (and one of those says the product is out of stock, when they've actually got plenty of copies with different cover art).

Why is this such a problem? Because multiple pages targeting the same phrases:

• Force the search engines to choose which page is the most "authoritative" or relevant to that subject on your site. In the case of Barnes and Noble, if the "out of stock" version shows up first, potential buyers will be very unhappy.
• Create additional competition for rankings - you're vying against yourself for position in the SERPs.
• Dilute the potential ranking ability of a single phrase by spreading link power, keyword targeting, and anchor text on your site across multiple pages.

There isn't a hard and fast rule that I apply to keyword self-cannibalization - sometimes it's important to have multiple pages with the same phrase in the title tag. I would, however, recommend that you take a careful look at your keyword targeting and determine if there's areas where a 301, a title tag change, or a better information architecture could create a more search-friendly and user-friendly site.