We often place redundant content and links on the websites we design, especially on the home page and in the sidebar and footer. We think this is smart, so long as it suits the goals of the site. Redundancy can also very easily be a bad thing. Here are some examples, both good and bad.
Home pages: we often place 'teaser' content from other sections of the site (about us, blog, etc.) on the home page. This acts as a lead-in for the various sections of your site, and offers visitors an overview of the content available on your site. These lead-ins are always accompanied by a button or link to their respective section of the website: 'read more about us', 'visit our blog', etc.
We also tend to place key information and links in multiple places throughout the site, especially in the header, footer, and sidebar. Important information like your company phone number or physical address should be easy to find, no matter what part of the site your user is on. Furthermore, the 'call to action' buttons on your website should be prominent and easy to find on every page. For example, if your goal is to generate inquiries, it's beneficial for your visitors to see 'contact us' in multiple places. It's important not to go overboard; you might irritate or annoy your visitors. The goal is always to improve the functionality of the site and make it as user-friendly as possible.
Another consideration is varying user behavior. We (almost) always have 'home' as the first link in the main navigation. We also make the logo a link to the home page. It isn't necessary to have two home page links side by side, but it doesn't hurt anything, and it accommodates a broader audience; some people are conditioned to click the logo, others will be looking at the main navigation.
These examples explain why we prefer to err on the side of 'good' redundancy. Bad redundancy equates to poor organization of your site's content. Or, it could simply be anything that is confusing or unclear. For instance, it doesn't make sense to place a 'contact us' button on your contact page. Similarly, if a user is browsing your online store, they don't need to see a 'shop now' button. However, you probably want several 'buy now' and 'view cart' buttons placed strategically on those pages.
All of this to say, when placing key information and links throughout your site, it is very important to consider the goals of the website, as well as how your visitors are going to interact with the site. With such a high saturation of websites and available information, it is more important than ever to make your website as clear and easy to use as possible.