Page names need to comply with standard filename rules. Page names can be repeated within the sitemap, however, there are benefits to having unique page names when using features like Import.
There are several ways to change the name of a page in the sitemap.
Similar to renaming files in Windows Explorer, performing a slow double-click on a page name in the Sitemap will all you to edit the page name directly in the sitemap.
Right-click on a page and choose Rename to open the name dialog.
Selecting a page in the sitemap and typing F2 will also allow you to edit the page name directly in the sitemap.