In the first box displayed, click 'Copy' beside 'Operations'. This will open up a copy of the page in edit mode. By default, Confluence will name the page as 'Copy of Pagename'.
Rename the page and make any other changes required.
If you need to move the new page to a different space or a different parent, click the 'EDIT' link (to the right of 'Location', just below the page's name), then select the new space and/or parent.
Click 'Save'.
Copying a page will duplicate all of the original page's attachments, but will not copy comments or labels from the original page.