When you add some modules to a page, you will have the option to find existing content, as well as creating new content. You can use find existing to search all the content on your site for the piece of content you would like to use. Adding content this way will not remove it from its current location, it will be added to this module in addition. If you change the content here, it will be changed anywhere else it appears on the site.
After clicking find existing you will be taken to a page with a search box on the left. Enter the name of the content into the search box, or leave it blank to search for everything. You can choose the page(s) you want to search or choose all to search the entire site. Then click search.
A list of all the content matching your search will now be displayed. Tick the box next to the content you want to relate.
