I have since found a much simpler way to duplicate the tab within Safari 4!
Go to 'Preferences' and under 'General', go to 'New tabs open with' and select 'Same Page'.
You can do the same to with new windows.
The good news is that this appears to have the same effect as the 'Clone Window' add-on I mentioned for Firefox, in that it does more than just copy and paste the URL into a new window, and actually enables you to go back in the new tab to previous pages (as if you were still on the original tab).
Hope this helps all! I must use this essential browser feature tens of times per day!

Thanks djthomas -- that's the answer! I came upon this thread because I had the same problem as the original poster, and your solution is the one that accomplishes what we need. The resulting tab is indeed cloned, including the history of the original tab. Perfect.

another reason to duplicate a tab & how to duplicate a window in safari
 

thank you for the 2 suggestions to duplicate tabs in safari, they worked like a charm. (hitting command and the back button, and also opening a new tab and dragging the icon at the left of the url in the address bar to the tab.)

i use duplicate tabs constantly. here's one reason; if i am shopping online and want to view 2 items, i can open a duplicate tab, hit the back button and pull up the other item. then i can toggle between the 2 tabs and see both items at once. there are many other reasons but this is one i can think of now. you can also use a duplicate window, which you can set up in safari preferences. with duplicate window, you can hit back button and pull up a previous page, and diminish the windows so you can see them side by side.

to create a duplicate window in safari go to: safari>preferences>general>new windows open with: and choose "same page" in the drop down.

in firefox for mac you can create duplicate windows and tabs with an add-on called tab mix plus. if you use this, make sure to set the tab mix plus options to duplicate page rather than current page. with duplicate page, the current page plus all the history will duplicate, so you can use the forward and back button.