Normally, you can simply right-click on the tab and use the Duplicate item on the menu to make a copy of the tab, but there’s no easy way to do this from the keyboard (that we know of). What you can do instead is use a shortcut key combination to highlight the location bar, and then create a new tab from that URL.
Just use the Alt+D shortcut key to put the focus into the address bar, and then use Alt+Enter to open that URL in a new tab. The trick is that you don’t have to move your thumb off the Alt key—just push down Alt, then hit D and Enter in quick succession to duplicate the current tab in a new tab. Readers will note that this method also works in Firefox, and duplicates the tab without duplicating the tab history — probably saving some memory usage while you are at it.
Stupid Geek Tricks: Duplicate a Tab with a Shortcut Key in Chrome or Firefox [How-To Geek]