Near as I can tell, duplicate will only work on simple objects or groups with simple objects. Complex figures with lots of polys and interconnected bone structures won't dup.

I used the following technique to put together a story series with recurring characters. Put your M4 and all his clothing and gear in a group with an appropriate name. Then highlight the group and do a File -> Export. It will save the highlighted group only as a Carrara file that can then be imported into other Carrara scenes. You can even import the same character multiple times in the same scene - but the poly count will probably go through the roof so be careful doing this.