Make sure you have a 404 Error Page set up. This is the page that your website serves for any request for any URL that doesn’t lead to any file or page. This acts as a “catch all” for pages you remove from the site. Your 404 Page should fit within the brand of your site, and often they are whimsical, to relieve any stress a visitor might experience from not finding what they are looking for. If you would like to test if your site has a 404 Page now, simply type in your domain followed by any random string of characters. For example, you can see ours at http://BooksAndWhatnot.com/KentonIsReallyAmazing, because the page doesn’t exist. (But maybe it should be). Since this step is a bit technical, ask your web developer to help you.
Next, update, delete, or unpublish any page that is no longer accurate or relevant. If you don’t have anything to fill the void, your 404 Page will now carry the load. (This solves example #1.)
I could write a volume on the negative effects of duplicate pages with different URLs. The short version is that search engines have a hard time deciding which version is the “original.” The easiest way to provide that insight is to add a little bit of code, called the “canonical tag,” to each page or to your template. By placing your preferred URL in this tag, you’re letting search engine indexing spiders know which URL you prefer they index and present in results. You may need your web developer’s help here again. If you’re interested there is much more information is available on this. Here’s an example of the tag: <link rel="canonical" href="http://www.example.com/product.php?item=swedish-fish" />
If you have a new page that replaces an old page, but the new page exists at a new URL, then the answer is a redirect. Again, there are multiple ways to skin this particular feline, however I prefer the .htaccess method. This method uses a file on your web server that acts as a map to visitors. A visitor requesting the old URL is magically transported to the new URL without any action on their part. While this is the best solution, changes in your .htaccess file could bring down your site completely, so it would be best if this was done by someone with the technical know-how. One of my favorite resources for you to pass on to that person is htaccessredirect.net