Because of simple's hierarchal page structure, it's not possible to have a page appear in two places. However, you can make it appear that way in the menu by using the Redirect URL property.
Go to one of the places where you'd like to have this page and create the page like any normal page. When you're done, publish the page and write down the PageID.
Go to the place in the site where you'd like the page to appear again. Create a page there but don't put any content on it.
In the Redirect URL field, type /pages/n.php where n is the PageID you copied in Step 1. Publish the page. Now when you click on the second link in the menu, it will redirect to the first.
If you insist on duplicating the content without redirecting users you can use common funcitons to copy the page body from the original. Follow steps 1 and 2 as above, but replace Step 3 with the following:
Click on the link to "Edit Without Xinha". In the body textarea, type the following:
<?PHP echo GetPageBody( n , $CurrentUserID);?>
When you're done, click Publish.
Note: if you put PHP tags on a page, you cannot edit that page with Xinha. Also, if you copy a page's contents with GetPageBody, none of the PHP tags on that page will be executed.