If you right-click the movieclip on the stage, you can select 'Duplicate symbol'. This will create a copy, however, any child movieclips will be shared with the original, so any edits to child movieclips will be reflected in the original movieclip.
To get around the above issue, if it's an issue for you then do this:
- In the library, right-click the movieclip and choose 'copy'
- Open a new FLA and paste the movieclip into the new library. All the associated movieclips and assets will be copied too.
- Create a new folder and place everything inside it. This folder can now be copied back to the original FLA without overwriting any of the original movieclips.