I believe that Tetsujin has the best answer (see above).
I read the above tips and tried several. Not noticing his tip at first I accidentally found the same solution, but for it to work this way you must have Safari's preferences set appropriately.
I am using El Capitan, OS X 10.11.1 and Safari 9.01. This tip works because we previously 1) opened Safari's Preferences dialog (Cmd/Comma), 2) selected the Tabs tab, 3) set the "Open pages in tabs instead of windows" drop-down list box to Automatically and 4) enabled the three check boxes just below that. What this does is mostly self-explanatory - text in the dialog box tells what each choice means.
Cmd/L, Cmd/Return - that's the entire tip!. Simple isn't it?
Cmd/L means Open Location and has the effect of opening the current URL in Safari's Smart Search field and selects it.
Cmd/Return means Open a page in a new tab. The URL in the Smart Search field opens in a new tab.
You can type this as 1) press Cmd and hold it while 2) you press L then 3) release L and press Return. You then 4) release both the Cmd and Return keys and you are done.
These two sets of keypresses will create a duplicate of the tab you are on and make it active.