Mac:
1. Go to finder.
2. Go to your Library.
3. Go to application support.
4. Go to minecraft folder.
5. Go to saves.
6. Find your world.
7. Right click and hit duplicate.
8. It should say (name) copy.
9. You now have a copy.
10. Go play minecraft and on the bottom by the date last played it should say (name) copy.
11. That world is the copy the one that says just (name) on the bottom by the datte last played.
12. Good job you did it right enjoy!