Open the same PDF multiple times

In Adobe Reader XI and Adobe Acrobat Reader DC

• Open your document;
• Go to the Window => New Window.

Now the first window should be duplicated.

Slight addition to serhio's answer.

In Acrobat Reader DC (released in 2015)

• Open your document;
• Go to the Window => New Window;
• Drag and drop the tab of the new window on to the tab bar of the old one.

Then you have two tabs for the same PDF file.

I think it's a great omission on the part of Adobe not to include a "Duplicate Tab" feature.

In acrobat reader 9 - To open the same document in a new tab in the same window, right click the tab, and choose Duplicate Tab then detach tab ( it will open in another window )